THE  FEEDING  OF 
CROPS  AND  STOCK 


A.D.  HALL 


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University  of  California. 


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THE    FEEDING    OF    CROPS    AND 
STOCK 


THE  FEEDING  OF 
CROPS  AND  STOCK 

AN      INTRODUCTION      TO      THE 

SCIENCE    OF    THE    NUTRITION    OF 

PLANTS  AND   ANIMALS 

BY   A.   D.    HALL,   M.A.,   F.R.S. 

DIRKCTOB  OF  THE   ROTHAMSTKD   EXPERIMENTAL  STATION 

FOBEION    MEMBER    OF    THE    ROYAL    ACADEMY    OF    AGRI- 

CITLTURE  OF  SWEDEN 

WITH  ILLUSTRATIONS  AND  DIAGRAMS 


NEW   YORK 
E.    P.    DUTTON   AND    COMPANY 

1911 


n 


n/ 


immM^Z  FilKO 


INTRODUCTION 

In  this  book  I  have  endeavoured  to  provide  a  general 
introduction  to  the  science  of  growing  crops  and  feeding 
animals :  an  outline,  in  fact,  of  the  theory  of  the  nutri- 
tion, first  of  the  plant  and  then  of  the  animal.  The  book 
is  intended  on  one  side  to  give  the  student  of  agriculture 
a  general  framework  of  ideas,  before  he  enters  upon  the 
more  detailed  study  of  agricultural  chemistry  that  will 
be  presented  to  him  by  his  professor.  I  have  found  that 
such  students  often  experience  a  difficulty  in  following  and 
making  the  best  use  of  their  lectures  because  the  whole 
trend  of  the  subject  is  strange  to  them  ;  they  would 
be  better  able  to  appreciate  the  value  of  the  illustrations 
they  receive  if  they  could  at  once  see  their  bearing  upon 
the  scheme  of  nutrition  of  plant  or  animal.  There  are 
many  students  also  in  our  schools  and  colleges  who  do 
not  as  a  rule  pursue  the  subject  any  further  than  it  is 
carried  in  these  pages ;  both  on  their  account  and  for 
the  student  who  is  only  laying  a  foundation,  I  have 
insisted  as  far  as  possible  on  the  main  principles  govern- 
ing this  branch  of  science,  hoping  thereby  to  induce  a 
sound  way  of  thinking  that  will  not  prove  misleading  on 


vi  INTRODUCTION 

further  study  and  practical  experience.  The  writer  of 
an  elementary  book  must  always  adopt  short  views  and 
make  dogmatic  statements  which  he  knows  to  be  no 
more  than  approximations  to  the  truth.  I  can  only  hope 
that  the  constant  reference  to  experiment,  and  the  whole 
style  in  which  the  book  has  been  written,  will  keep 
before  the  reader  an  appreciation  of  how  much  still 
remains  unsaid. 

Even  more  than  for  the  student,  this  book  is  intended 
for  the  workaday  farmer  who  wants  to  get  an  intelli- 
gent conception  of  how  his  crops  and  stock  make  their 
growth.  Complex  and  unknown  as  many  of  these 
processes  are,  the  main  outlines  are  sufficiently  estab- 
lished to  have  a  bearing  upon  practice  ;  but  the  practical 
application  can  be  more  surely  and  readily  made  if  the 
farmer,  with  his  inside  knowledge  of  the  way  things  can 
be  done,  will  learn  the  theory,  than  by  any  attempt  of  the 
scientific  man  striving  from  the  outside  to  get  up  the 
working  conditions. 

Moreover,  the  subject  is  interesting ;  it  should  neither 
be  difficult  nor  tedious  for  the  man  with  an  ordinary 
education  to  learn  how  a  plant  draws  its  nutriment 
from  the  soil,  and  how  the  animal  depends  in  its  turn 
upon  the  plant ;  so  if  this  book  is  not  readable,  mine  is 
the  fault,  and  I  have  failed  in  the  object  with  which  I 
set  out.  To  this  end  I  have  avoided  technical  language 
as  much  as  possible,  and  I  have  assumed  that  the  reader 
possesses  little  or  no  knowledge  of  chemistry.  Of 
course,  a  book  of  this  kind  cannot  be  finally  intelligible 
unless  the  reader  possesses  some  ideas  about  the  con- 
stitution of  air  and  water  and  the  nature  of  chemical 


INTRODUCTION  vii 

change,  but  these  ideas  are  becoming  more  generally 
diffused  every  day,  and  the  unfamiliar  reader  may  to 
some  extent  gather  them  as  he  goes  along.  I  must 
express  my  indebtedness  to  Dr  E.  J.  Russell  for  his 
advice  on  certain  sections  of  the  book,  and  to  Miss 
W.  E.  Brenchley  for  the  botanical  drawings  therein  con- 
tained. Miss  L.  M.  Underwood  also  has  greatly  helped 
me  by  compiling  the  index.  As  on  other  occasions 
also,  I  have  to  thank  Mr  G.  T.  Dunkley  for  the  care  he 
has  taken  in  looking  through  the  proofs,  and  verifying 
the  results  quoted. 

A.  D.  HALL. 


The  Rothamsted  Experimental  Station, 
June  1910. 


TABLE  OF  CONTENTS 

CHAPTER  I 

WHAT  THE   PLANT   IS   MADE   OF 

PAQB 

Elements  present  in  the  Plant.  The  Chief  Compounds  form- 
ing Plant  Tissues — Carbohydrates,  Fats,  Proteins,  a-Fro- 
teins,  and  Ash.  The  Growth  of  a  Plant  from  a  Seed ; 
the  Embryo  and  the  Food  Store.  Conditions  necessary 
for  Germination — Water,  Warmth,  Air.  How  Materials 
are  moved  to  the  Growing  Parts  of  a  Plant.  Seed 
Testing       .......  I 

CHAPTER    II 

THE   WORK   OF   THE   LEAF 

The  Increase  of  Weight  in  a  Growing  Plant  is  derived  from 
the  Air.  Plants  split  up  Carbon  Dioxide  and  give  off 
Oxygen.  Purification  of  the  Air  by  Plants.  Formation 
of  Starch  in  the  Green  Leaf.  Motive  Power  supplied 
by  Light  All  parts  of  the  Living  Plant  are  also 
breathing.  Necessity  of  Leaves  to  the  Growth  and 
Ripening  of  the  Plant.  The  Leaves  of  the  Plant  give 
off  Water.     How  much  Rainfall  is  required  by  Crops    .         19 


X  CONTENTS 

CHAPTER  III 

THE  WORK  OF  THE  ROOTS 

PAGB 

The  Roots  as  anchoring  the  Plant.  The  Roots  supply  the 
Plant  with  Water.  Roots  require  Air.  Roots  can  only 
take  in  dissolved  Material.  Etching  Action  of  Roots 
due  to  their  Excretion  of  Carbon  Dioxide.  Elements 
necessary  to  the  Nutrition  of  Plants.  Plants  require 
Combined  Nitrogen  .....        42 


CHAPTER   IV 

CHANGES  OF  COMPOSITION   WITHIN   THE 
PLANT 

The  Manufacturing,  Resting,  and  Spending  Stages  in  a 
Plant's  Development.  The  Course  of  Nutrition  and 
Migration  in  the  Growth  of  Wheat.  The  Ripening  of 
the  Grain.  Storage  and  Migration  in  Root  Crops. 
Removal  of  Food  Materials  from  the  Leaves  of  Trees 
as  they  Ripen.  The  Ripening  of  Fruit.  Effect  of  Soil 
and  Climate  upon  the  Composition  and  Quality  of  the 
Crop  .......         59 


CHAPTER  V 

THE  ORIGIN   AND   NATURE  OF  SOILS 

The  Weathering  of  Rocks  to  Soil.  Solution  of  Rock 
Materials  in  Water  containing  Carbon  Dioxide. 
Action  of  Frost.  Transport  of  Soil  by  Rain  and  Run- 
ning Water.  Action  of  Worms.  Approximate  Analysis 
of  Soils.  Properties  of  Clay  and  Sand.  Chemical 
Constituents  of  Soils.     Soils  and  Subsoils  ,  »        78 


CONTENTS 


CHAPTER  VI 

CULTIVATION   AND  THE  MOVEMENTS  OF  SOIL 
WATER 

PAQB 

Nature  of  the  Film  of  Liquid  surrounding  Wet  Particles  of 
Soil.  Retention  and  Movements  of  Water  due  to 
Surface  Tension.  Percolation.  Rise  of  Subsoil  Water 
by  Capillarity.  Value  of  Autumn  Ploughing.  Effect 
of  Spring  Cultivations.  Cooling  of  the  Land  by 
Evaporation.  Effect  of  Hoeing  and  Rolling  upon  the 
Temperature  and  Water-content  of  the  Soil.  Dry 
Farming  in  Semi-arid  Regions.  Drainage  and  the 
Temperature  of  Soils.  Spring  Frosts.  Early  and 
Late  Soils lOO 


CHAPTER  Vn 

THE  LIVING    ORGANISMS  OF  THE  SOIL 

Formation  of  Nitrates  in  the  Soil.  Bacteria  in  the  Soil 
which  decompose  Organic  Matter.  Bacterial  Loss  of 
Nitrogen  from  the  Soil.  Formation  of  Humus.  Fixation 
of  Nitrogen  by  Bacteria  associated  with  Leguminous 
Plants.  Value  of  Clover  Crops  in  the  Rotation.  In- 
oculation of  Soil.  Other  Bacteria-fixing  Nitrogen  in 
the  Soil.  Accumulation  of  Nitrogen  in  Virgin  Soils. 
Dependence  of  Soil  Fertility  upon  Bacteria        .  .120 

CHAPTER   Vni 

THE  CHEMICAL  COMPOSITION  OF  THE  SOIL 

Plant  Food  found  in  Normal  Soils.  Dormant  and  Available 
Plant  Food.  Rotations  and  Plant  Food  in  the  Soil. 
Systems  of  Farming— Wasteful  and  Conservative. 
Requirements  of  Different  Crops  for  Fertilisers.  Types 
of  Soil— Characteristic  Weeds  and  Crops  .  .      I49 


xii  CONTENTS 

CHAPTER    IX 

FOODS 

PAOI 

Composition  of  Cattle  Foods.  Nature  of  Carbohydrates, 
Fat,  Proteins,  Fibre,  Ash.  Processes  of  Digestion  in 
the  Animal  Body.  Digestibility.  Character  of  various 
Concentrated  Foods,  Cereals,  Roots,  Straw,  and  Hay. 
Valuation  of  Feeding  Stuffs         .  .  .  .169 

CHAPTER   X 

THE   UTILISATION   OF   FOOD   BY  THE   ANIMAL 

Food  as  a  Source  of  Energy.  Heat  Value  of  various 
Foods.  Energy  consumed  in  Digestion  and  In- 
ternal Work.  Maintenance  Rations.  Feeding  for 
Rapid  Work  or  Increase  of  Weight.  Amount  of  Food 
required  for  a  given  Amount  of  Work.  Nitrogenous 
Materials  required  to  repair  Tissue  Waste.  Minimum 
of  Protein  necessary  .  .  .  .  .185 

CHAPTER   XI 

FOOD   REQUIRED   BY  THE   GROWING   AND 
FATTENING  ANIMAL 

Composition  of  Lean  and  Fat  Animals.  Food  required  to 
produce  a  given  Increase  of  Live  Weight.  Starch 
Values  of  Foods.  Albuminoid  Ratio.  Food  Rations 
for  various  Purposes,  based  upon  Starch  Values  .       207 

CHAPTER   XII 

FARMYARD  MANURE 

Composition  of  Animal  Excretions.  Litter.  Changes 
taking    place    during    the    Making    and    Storage    of 


CONTENTS  xiii 


Manure.  Losses  of  Nitrogen  in  Manure-making — 
Unavoidable  or  due  to  wasteful  Methods.  Composi- 
tion of  Farmyard  Manure  from  various  Sources.  Care 
of  Farmyard  Manure.  Farmyard  Manure  as  a 
Fertiliser.  Value  of  Farmyard  Manure.  Valuation  of 
Manure  Residues  derived  from  the  Consumption  of 
Purchased  Feeding  Stuffs.     Cost  of  Farmyard  Manure      224 


CHAPTER   XIII 
ARTIFICIAL  MANURES  AND  FERTILISERS 

Nature  of  a  Fertiliser.  Fertilisers  containing  Nitrogen — 
Nitrate  of  Soda,  Sulphate  of  Ammonia,  Soot — their 
Use  and  Value.  Fertilisers  containing  Phosphoric 
Acid.  Bones,  Superphosphate  or  Acid  Phosphate, 
Basic  Slag  or  Phosphate  Powder,  Ground  Rock 
Phosphate.  Potash  Fertilisers.  Guanos.  Industrial 
Residues.  Tankage.  Action  of  Fertilising  Ingredients 
upon  Crops.  Expenditure  on  Fertilisers.  Character  of 
Fertiliser  required  for  particular  Crops.  Valuation  of 
Fertilisers   .......      248 

CHAPTER  XIV 


Composition  of  Milk.  Variations  to  which  the  Composition 
of  Milk  is  subject.  Effect  of  Individuality,  Breed,  Food, 
Time  of  Milking,  Period  of  Lactation.  Feeding  for 
Milk.  Composition  of  Butter.  Nature  of  the  Churn- 
ing Process.  Effect  of  various  Foods  upon  the  Quality 
of  the  Butter.  Composition  of  Cheese.  Changes 
taking  place  during  the  Cheese-making  Process.  The 
Ripening  of  Cheese.  Importance  of  Cleanliness  in  all 
dealings  with  Milk  .  .  .  .  .       269 


Index    •...,...      291 


LIST   OF    ILLUSTRATIONS 

1.  Germinating  Seedlings  of  Beans  .  .  Face  page      6 

2.  Seeds  of  Barley  at  Various  Stages  of  Growth    .         „  8 

3.  Mint  Plant  in  Water  splitting  up  Carbon  Dioxide 

and  evolving  Oxygen    .            .            .            .         „  24 

4.  Apparatus  to    show    the    Absorption    of    Carbon 

Dioxide  by  Green  Leaves          .            .            •          »>  24 

5.  Starch   Formation   in   the  parts    of  an  Artichoke 

Leaf  exposed  to  Light  .            .            .            •         v  26 

6.  Barley  Seedlings  enclosed  within  Bottle            .         „  30 

7.  Fern  that  has  been  growing  inside  a  closed  Bottle 

for  Thirty-six  Years      ••••»}  32 

8.  Transpiration  of  Water  from  Leaf         .            .         „  35 

9.  Apparatus  for  Measuring  Transpiration  from  Leaf  „  36 
10.  Stomata  on  Lower  Surface  of  Sweet  Pea  Leaf  .  „  38 
I  \.  Root  Hairs  of  Barley       •            •            •            •          „  50 

12.  Contractile  Taproot  of  Dandelion          .            .          „  44 

13.  Slab  of  Polished  Marble  etched  by  Roots  of  Bean 

Plant      .......  50 

14.  Series  of  Plants  grown  in  Water  Cultures         .          „  52 

15.  Diagram  showing   Migration   of  Foodstuffs  with 

the  Grain  of  Wheat       •            .            .            .          „  64 


xvi  LIST  OF  ILLUSTRATIONS 

i6.  Formation  of  a  Sedentary  Soil    .  .  .Face  page    78 

17.  Formation  of  a  Drift  Soil  .  .  •  »  84 

18.  Experiment  to   show  Percolation  and  Absorption 

of  Water  by  Different  Soils      .  .  •  »  92 

19.  Photograph   illustrating  Liquid   Film   round   Soil 

Particles  ......  100 

20.  Diagram  illustrating  Capillary  Rise  of  Liquids  „  104 

21.  Distribution  of  Sun's   Rays   upon   Southerly  and 

Northerly  Slope  ....  Page     1 1 7 

22.  Apparatus  used  for  Experiments  with  Bacteria  Face  page     122 

23.  Diagram  showing  the  Energy  required  from  the 

Food  by  Oxen  of  Different  Weights    .  .  „  198 

24.  Diagram  showing  the  Composition  of  the  Carcass 

of  Lean,  Half- Fat,  and  Fat  Oxen        .  .  „  208 


THE  FEEDING  OF  CROPS 
AND  STOCK 

CHAPTER  I 

WHAT  THE  PLANT  IS  MADE  OF 

Elements  present  in  the  Plant.  The  Chief  Compounds  forming 
Plant  Tissues — Carbohydrates,  Fats,  Proteins,  ^-Proteins,  and 
Ash.  The  Growth  of  a  Plant  from  a  Seed  ;  the  Embryo  and 
the  Food  Store.  Conditions  necessary  for  Germination — 
Water,  Warmth,  Air.  How  Materials  are  moved  to  the 
Growing  Parts  of  a  Plant.     Seed  Testing. 

Before  we  can  obtain  any  idea  of  how  a  plant  feeds 
and  grows,  it  is  necessary  to  find  out  to  some  extent  of 
what  it  is  composed,  and  for  this  purpose  a  few  simple 
experiments  must  be  made.  Let  us  begin  by  taking 
some  green  leaves  of  any  plant,  a  carrot  or  a  potato  to 
represent  a  root,  and  some  wheat  or  maize  as  examples  of 
seeds  ;  weigh  out  portions  of  each  in  basins  of  porcelain, 
nickel,  or  (best)  of  platinum,  and  then  put  them  on  a 
sand  bath  or  over  a  low  flame  with  glass  plates  covering 
the  tops  of  the  basins.  In  a  very  few  moments  the 
glass  plates  will  be  dimmed  over  and  water  will  begin 
to  collect  in  drops,  water  that  has  been  driven  out  of 
the  plant  material  by  the  heat.  As  soon  as  you  have 
satisfied  yourself  that  the  plant  stuff  is  losing  water^ 
remove  the  glass  coverings  and  put  the  basins  in  the 
oven  to  dry  completely,  a  process  which  will  require  a 
day  or  so.  On  reweighing  the  dishes  it  will  be  found 
1  A 


2  WHAT  THE  PLANT  IS  MADE  OF         [chap. 

that  the  contents  have  lost  a  good  deal  of  water, 
amounting  to  80  to  90  per  cent,  of  the  original  weight 
of  the  green  leaves ;  the  roots  lose  nearly  as  much, 
but  the  seeds  only  10  to  15  per  cent. 

Water  is  an  invariable,  and  indeed  the  chief 
constituent  of  the  living  plant. 

Take  now  the  dry  plant  tissues  still  in  their  basins, 
and  heat  them  more  strongly  over  the  Bunsen  or 
Argand  flame,  covering  the  greater  part  of  the  basins 
with  a  sheet  of  metal.  Thick  gases  possessing  a 
pungent  smell  will  be  given  off,  and  these  after  a  time 
will  take  fire  if  allowed  to  come  in  contact  with  the 
lamp ;  after  a  time  extinguish  the  flame  by  completely 
covering  the  basin,  and  cease  heating  until  the  contents 
are  cool  enough  for  examination.  It  will  now  be  seen 
that  the  whole  interior  of  the  basin  is  covered  with 
black  soot,  and  that  the  plant  material  is  charred  or 
carbonised ;  there  is  abundant  evidence  that  the  black 
element,  Carbon^  has  been  set  free  from  the  original 
plant  tissues.  Now  resume  the  heating,  but  without 
any  covering,  push  it  as  rapidly  as  possible,  and  let  the 
contents  of  the  basins  burn  away  until  little  or  nothing 
of  the  black  carbon  is  left,  though  to  get  rid  of  the  last 
traces  it  may  be  necessary  to  put  the  basins  in  a 
muffle  furnace  and  raise  the  contents  to  a  bright  red 
heat.  At  the  end,  when  everything  possible  has  been 
burnt  away,  there  will  still  be  found  a  little  grey  or 
white  ash,  which  on  weighing  will  amount  to  from  2  to  5 
per  cent,  of  the  dry  matter  that  was  left  after  the  water 
had  been  driven  off.  In  the  plant  ash  only  a  few 
elements  are  to  be  found,  but  these  are  the  same  what- 
ever the  plant  or  wherever  it  has  been  grown,  with  a  few 
exceptions  that  are  small  and  unimportant.  By  tests 
which  need  not  here  be  detailed,  we  can  always  demon- 
strate in  the  plant's  ash  the  presence  of  the  elements 


I.]  ELEMENTS  PRESENT  IN  PLANTS  3 

phosphorus,  sulphur,  and  to  a  smaller  degree,  chlorine. 
Similarly  we  shall  always  find  five  metallic  compounds, 
— potash,  soda,  lime  (oxide  of  calcium),  magnesia,  and  a 
smaller  quantity  of  iron ;  when  we  are  dealing  with 
cereals  or  grasses,  there  will  also  be  a  large  proportion 
of  silica  in  the  ash,  though  it  is  absent  or  only  found  in 
very  small  quantities  in  the  ash  of  other  plants.  Thus 
we  distinguish  in  the  plant,  (i)  water;  (2)  a  combustible 
part,  of  which  carbon  is  the  base ;  and  (3)  a  small  pro- 
portion of  incombustible  mineral  ash.  There  remains 
another  element  which  disappeared  with  the  combustible 
portion,  but  which  belongs  neither  to  the  carbon  nor  to 
the  hydrogen  and  oxygen  with  which  the  carbon  are 
mainly  combined.  To  detect  it  we  must  mix  some  of 
the  dry  plant  material  in  a  test-tube  with  lime,  or 
better  with  the  mixture  called  soda-lime,  and  then 
heat  the  whole ;  gases  will  soon  be  given  off  in  which 
we  can  detect  the  familiar  smell  of  ammonia  or  hartshorn. 
Now  ammonia  is  a  compound  of  the  element  nitrogen, 
and  ammonia  is  as  a  rule  liberated  from  any  compound 
of  nitrogen  with  carbon  and  its  associated  elements, 
hydrogen  and  oxygen,  when  the  compound  is  heated 
with  lime  or  other  alkali.  A  proper  analysis  will 
show  that  only  i  to  2  per  cent,  of  the  dry  matter  of 
the  plant  consists  of  nitrogen,  but  it  is  an  indispensable 
element  and  always  found  in  the  plant. 

To  illustrate  more  definitely  what  may  be  expected 
in  the  plant  and  in  what  proportions,  in  Table  I.  is  set 
out  a  list  of  the  quantities  of  the  various  elements  found 
in  the  grass  cut  from  an  average  acre  of  land  at  the 
time  it  was  ready  for  mowing,  this  example  being 
chosen  because  it  gives  a  composite  analysis  represent- 
ing a  large  variety  of  plants.  The  analys"s  is  set  out 
both  in  pounds  per  acre,  and  as  percentages  of  the 
whole  plant  and  of  the  dry  matter  respectively. 


4  WHAT  THE  PLANT  IS  MADE  OF        [chap. 

Table  I.— Composition  of  Meadow  Grass  from  One  Acre. 


Lb.  per  Acre. 

Percentages 

Of  whole  plant. 

Of  dry  matter. 

<l> 

Is 
is 

o 

Water       . 
'Carbon      . 
Hydrogen 
Nitrogen  . 
Oxygen    . 
Sulphur    . 

'Phosphoric  Acid 

Sulphuric  Acid          . 

Chlorine  . 

Silica        .        . 

Potash      . 

Soda         .         .        . 

Magnesia . 

Lime 

Oxide  of  Iron  . 
^Sand,  etc. 

8,378 

1,315 

144 

49 
1,090 

15 

13 
II 
16 
57 
56 
12 
10 
28 
I 
5 

74-8 

117 

1-3 

0.4 

9-7 
o«i 

O-I 
O-I 
O-I 

0-5 
0-5 

O-I 
O-I 

0-31 

46-6 
5-1 
17 

38-6 

0-5 

0-5 
0-4 
0.6 

2'0 

2-0 

0-4 
0-4 

I-O 

0-04 

0-2 

11,200 

99.8 

100-04 

So  far  we  have  only  been  dealing  with  the  elements 
common  to  all  plants,  which,  as  will  be  seen  later,  are 
also  common  to  animals  as  well ;  but  the  elements  are 
only  the  raw  material — bricks,  stones,  beams,  mortar, 
etc. — out  of  which  all  sorts  of  different  edifices  can  be 
constructed.  There  exist  in  fact  in  the  plant  material 
certain  characteristic  groups  of  compounds,  the  members 
of  each  group  being  built  up  in  much  the  same  way  but 
quite  differently  from  the  members  of  any  other  group, 
although  only  the  same  elements  may  have  been  used 
in  the  building  of  both  groups.  For  example,  the  fats 
form  a  definite  class  of  bodies  common  in  the  seeds  of 
plants ;  they  are  easily  recognisable  and  are  distinct  in 
every  outward  property  from  the  sugars,  another  natural 
group  of  substances  found  in  plants.  Yet  both  fats  and 
sugars  contain  the  same  elements — carbon,  hydrogen. 


I.]        PROXIMATE  CONSTITUENTS  OF  PLANTS         5 

oxygen — and  no  other;  the  difference  between  them 
arises  from  certain  differences  in  the  proportions,  and 
particularly  in  the  manner  of  building  up  the  structure. 
Moreover  there  are  particular  natural  agencies  by  which 
one  class  of  bodies  can  be  transformed  into  the  other, 
just  as  a  house  might  be  taken  down  and  rebuilt  from 
the  same  materials  in  a  totally  different  style. 

Among  the  more  important  of  these  universal 
constituents  of  plants  we  may  place  first  of  all  the 
carbohydrates,  a  great  group  of  substances  so  called 
because  in  them  the  elements  carbon,  hydrogen,  and 
oxygen  are  combined  together  in  the  proportions  that 
suggest  a  combination  of  carbon  with  water.  The 
carbohydrate  group  includes  the  sugars,  so  easily 
recognisable  in  the  sugar-cane  and  the  beetroot,  but 
also  detectable  in  fruits  (a  raisin  is  most  easily  tested), 
and  by  special  tests  in  the  leaves  and  other  parts  of  the 
living  plant.  Then  come  the  starches,  which  are  readily 
washed  out  of  flour,  potatoes,  and  many  other  storage 
organs  of  plants ;  also  certain  gums  and  mucilages, 
which  are  closely  related  to  the  starches.  Finally,  we 
can  include  the  celluloses  and  fibres,  which  we  obtain  in 
a  very  pure  state  in  the  simple  vegetable  cells  con- 
stituting cotton  and  linen,  and  less  pure  in  other  fibres 
such  as  hemp  and  jute,  and  in  wood  itself 

Rather  more  concentrated  than  the  carbohydrates, 
/>.,  containing  still  only  carbon,  hydrogen,  and  oxygen, 
but  with  a  higher  proportion  of  carbon,  are  the  fats, 
oils,  and  waxes  which  are  present  in  many  vegetable 
tissues ;  but  only  in  the  seeds,  nuts,  and  fruits  of  certain 
plants  is  there  enough  to  be  squeezed  out  for  commercial 
purposes.  The  seed  is  always  a  very  concentrated 
storehouse  of  food  for  the  future  plant,  and  it  is  in  the 
seed  that  such  concentrated  materials  as  oils  and  fats 
are  mainly  found. 


6  WHAT  THE  PLANT  IS  MADE  OF         [chap. 

So  far  the  compounds  have  only  contained  carbon, 
hydrogen,  and  oxygen,  but  in  all  plants  there  is  another 
group  of  compounds  built  up  as  before  with  carbon 
as  the  centre,  but  with  nitrogen  also  as  part  of 
the  fabric.  Of  these  compounds  containing  nitrogen 
the  most  important  are  the  proteins  (in  older  phraseology 
proteids  or  albuminoids) — complex  bodies  containing 
carbon,  nitrogen,  hydrogen,  oxygen,  and  smaller  propor- 
tions of  sulphur  and  phosphorus.  Perhaps  the  easiest 
of  these  bodies  to  separate  is  gluten  from  wheat  flour, 
but  by  suitable  tests  they  may  be  recognised  in  all 
plants  and  in  all  parts  of  the  tissues.  The  proteins  are 
very  elaborately  constructed  bodies,  and  are  either 
insoluble  or  not  properly  soluble  in  water  ;  but  as  a  sort 
of  intermediate  stage  in  their  building  up  or  breaking 
down,  come  certain  simpler  soluble  compounds  of  carbon, 
hydrogen,  oxygen,  and  nitrogen,  often  called  amides, 
though  the  name  is  not  very  correct,  and  it  will  be 
simpler  to  call  them  ^-proteins. 

Of  course  the  carbohydrates,  the  fats,  the  proteins 
are  not  the  only  groups  of  compounds  occurring  in  plant 
materials :  there  are  also  bodies  like  the  essential  oils, 
which  give  scent  to  plants,  the  resins,  the  vegetable 
acids,  the  bitter  and  poisonous  principles,  etc.,  etc.;  but 
the  three  groups  enumerated  constitute  by  far  the 
greater  part  of  every  plant,  and  on  them  only  depends, 
at  least  in  its  broad  outlines,  the  life  of  the  plant  and  in 
its  turn  of  the  animal. 

Since  the  crop  starts  with  the  seed,  we  must  begin 
by  finding  out  how  a  seed  is  constructed  and  what 
conditions  are  necessary  to  make  it  germinate  and  grow 
into  a  plant  capable  of  yielding  seed  in  its  turn.  Take 
some  conveniently  large  seeds — beans  and  maize  form 
the  best  examples,  though  they  may  be  supplemented 
or    replaced  by  sunflowers  and  wheat — soak  them  in 


III. 


Torn  Testa... 
Cotyledon  . . . 

Plumule )] 

Radicle 


II. 


Testa 


..  cotyledons 


Fig.  I.— Germinating  Seedlings  of  Beans, 

I.  Seed  split  open  after  germinating. 
II.  Root  a  few  days  old. 
III.  Still  older  seedling  showing  leaves  and  secondary  root?. 


[Face  page  6. 


I.]  STRUCTURE  OF  THE  SEED  7 

water  for  a  few  hours,  surround  them  with  damp  sawdust, 
and  then  keep  them  in  a  warm  place  for  a  day  or  two  in 
order  to  start  their  germination.  As  soon,  however,  as 
they  have  been  well  soaked  they  may  be  pulled  to  pieces, 
and  the  function  of  each  part  can  be  followed  up  day 
by  day  as  the  growth  advances.  Taking  first  the  bean, 
v/e  begin  by  finding  a  tough  outer  skin,  which  after 
soaking  can  be  peeled  off  entirely ;  when  the  plant 
germinates  under  natural  conditions,  this  skin  is  burst 
and  thrown  off.  Under  the  skin  we  find^the  mass  of  the 
seed,  which  splits  at  once  into  two  flat  discs,  united  at 
one  point  by  a  small  structure,  that  for  the  moment  we 
may  liken  to  a  hinge.  Notice  the  position  of  this  hinge 
with  regard  to  the  markings  in  the  seed  coat  which 
indicate  where  the  bean  was  attached  to  the  pod,  and 
therefore  to  the  parent  plant.  The  meaning  of  the 
structure  of  the  bean  will  only  become  plain  by  following 
up  the  successive  stages  in  its  growth.  It  will  then  be 
seen  that  what  we  have  called  the  hinge  develops  into 
the  future  plant ;  it  puts  out  a  root,  and  in  the  opposite 
direction  it  elongates  into  a  shoot,  from  the  tip  of 
which  leaves  begin  to  unfold — it  is  indeed  the  "  embryo," 
the  seat  of  life  in  the  plant.  The  two  flat  discs  which 
are  united  by  the  embryo  open  apart  and  set  them- 
selves at  right  angles  to  the  new  stem,  if  the  plant  is 
being  grown  in  the  ordinary  way  ;  in  the  open  air  they 
become  green,  but  at  the  same  time  they  slowly  shrink 
and  become  emptied  of  their  contents,  finally  they 
shrivel  and  drop  off.  They  are  known  as  "  cotyledons," 
or  seed  leaves.  In  the  case  of  the  maize  the  structure 
is  somewhat  different ;  at  one  end  of  the  grain  will  be 
found,  as  before,  the  embryo  from  which  proceed  the 
rootlets,  no  longer  a  single  one  but  several,  and  a  shoot 
which  shows  a  single  leaf.  The  greater  part  of  the  grain, 
however,  is  occupied  not  by  a  pair  of  seed  leaves,  but 


8  WHAT  THE  PLANT  IS  MADE  OF        [chap. 

by  a  mass  of  material  which  remains  attached  to  the 
side  of  the  stem  of  the  young  plant.  As  the  plant 
grows  the  seed  material  gradually  disappears,  until  after 
a  time  only  an  empty  husk  remains,  and  then  falls  off. 
In  the  case  of  maize  and  similar  seeds  like  wheat  and 
barley,  which  begin  by  throwing  up  only  a  single  leaf, 
this  attached  food  store  does  the  work  of  the  seed 
leaves  in  the  bean  and  similar  plants,  and  is  known  as 
the  endosperm.  Now  take  some  of  the  soaked  seeds 
and  mutilate  them  in  various  ways  before  setting  them 
to  germinate :  cut  out  the  embryo,  cut  bits  off  the 
embryo,  poison  it  by  touching  it  with  a  trace  of  carbolic 
acid  or  a  drop  of  boiling  water,  cut  off  one  cotyledon  or 
half  of  both  cotyledons,  cut  off  a  large  and  a  small  part 
of  the  endosperm. 

The  seeds  will  not  grow  at  all  if  the  embryo  is  cut 
out  and  killed  or  much  mutilated,  but  the  embryo  itself 
will  make  a  start  to  grow  without  cotyledons  or  endo- 
sperm, though  the  length  of  time  it  will  subsist  will 
depend  on  how  much  of  the  latter  has  been  left  to  it. 
The  embryo  is  in  fact  the  seat  of  life  in  the  seed, 
the  rest  (cotyledons  or  endosperm)  is  dead  food  material 
stored  up  by  the  parent  plant  to  give  sustenance  to  the 
new  plant  (embryo)  until  it  can  establish  itself  in  the 
world.  It  has  been  found  possible  to  grow  an  embryo 
removed  from  its  endosperm  on  blotting  paper  or 
similar  porous  material,  if  it  is  kept  moistened  with  an 
appropriate  food  solution  containing  sugars,  <3'-proteins, 
and  similar  plant  constituents. 

We  may  now  sum  up  the  facts  we  have  ascertained 
about  the  seed :  it  possesses  a  coat  of  one  or  more 
membranes,  an  embryo  (which  is  the  seat  of  the  life  of 
the  seed),  and  a  food  store  for  the  nutrition  of  the  embryo 
until  it  has  sufficiently  developed  to  feed  itself  on  the 
soil  and  the  air,  and  this  food  store  may  take  the  form 


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I.]  GERMINATION  9 

of  a  pair  of  cotyledons  or  a  single  endosperm.  There 
are  other  minor  differences  in  arrangement  that  need 
not  concern  us ;  there  are  also  very  interesting  variations 
in  the  way  the  root  develops,  in  the  origin  of  secondary 
roots,  side  buds,  etc.,  etc.,  and  again  in  the  character  of 
the  cotyledons  and  the  way  they  develop  above  ground, 
but  these  points  do  not  bear  directly  on  nutrition,  and 
the  student  must  follow  them  up  for  himself. 

We  may  now  proceed  to  examine  the  conditions 
which  favour  or  are  necessary  to  the  germination  of  the 
seed.  In  the  first  place,  moisture  is  required  ;  dry  seeds 
can  be  kept  a  considerable  time  without  apparent  change, 
it  is  only  when  the  embryo  has  absorbed  sufficient  water 
to  get  some  of  its  soluble  constituents  into  solution  that 
growth  can  begin  at  all.  Hence  a  seed  must  be  sown 
in  a  sufficiently  moist  soil  and  the  supply  of  moisture 
must  be  maintained  until  the  plant  is  established  ;  more- 
over, moisture  must  reach  the  embryo  inside  the  seed 
coat,  yet  in  some  cases  the  seed  coat  offers  a  considerable 
resistance  to  the  passage  of  water  through  it.  Every 
seed  possesses  a  fine  passage  through  the  seed  coat, 
called  the  micropyle,  but  in  many  cases  soon  after  the 
seed  is  ripe  the  micropyle  becomes  closed  by  the 
shrinkage  of  the  seed  coat,  which  itself  also  grows 
impervious  to  water.  Hence  follows  a  great  delay  in 
germination,  which  may  not  take  place  indeed  for  a 
year  or  longer  until  the  seed  coat  has  wholly  decayed 
away.  With  many  garden  plants,  e.g.  the  auricula  and 
the  hellebore,  germination  will  take  place  in  a  few  days 
if  the  seeds  are  sown  directly  they  are  ripe,  but  if  once 
they  are  dried  off  and  stored  before  sowing  they  will 
probably  remain  dormant  for  a  year  in  the  seed  pans 
and  then  begin  to  germinate  slowly  or  at  irregular 
intervals.  In  any  sample  of  clover  seed  a  certain 
number  of  "  hard  "  seeds   will   always  be  found  which 


lO 


WHAT  THE  PLANT  JS  MADE  OF        [chap. 


fail  to  germinate  when  the  usual  test  is  made,  although 
they  are  perfectly  formed  and  will  eventually  start  into 
growth  after  a  lapse  of  time.  In  some  of  these  cases 
where  the  seed  coat  is  obstinate  the  penetration  of 
water  may  be  hastened  by  various  mechanical  devices ; 
the  seed  coat  may  be  cut  or  filed  through  (gardeners  do 
this  with  Canna  seeds),  or  the  seeds  may  be  dipped  for 
a  moment  in  boiling  water,  or  even  soaked  for  a  short 
time  in  strong  sulphuric  acid,  which  is  then  quickly 
washed  away.  Certain  tree  seeds  only  germinate  after 
a  forest  fire  has  swept  over  the  ground ;  heat  has  been 
necessary  to  break  up  either  the  nut  in  which  the  seeds 
were  enclosed  or  the  seed  coat  itself  The  object  is  in 
all  cases  the  same,  to  begin  the  breakdown  of  the  seed 
coat  and  admit  moisture  to  the  embryo. 

Table  II.— Temperatures  required  by  Germinating  Seeds, 
(Fahrenheit.) 


Minimum 
limit. 

Optimum. 

Maximum 
limit. 

Degrees. 

Degrees. 

Degrees. 

Wheat  .... 

41 

81 

99 

Barley   .... 

41 

83 

100 

Mustard 

32 

83 

99 

Turnips 

46 

89 

109 

Cucumber 

56 

93 

115 

Besides  requiring  a  provision  of  moisture,  seeds  will 
only  germinate  within  certain  well-defined  limits  of 
temperature ;  for  each  species  there  is  a  minimum 
temperature  below  which  no  germination  will  take 
place,  above  that  minimum  the  rapidity  of  germination 
increases  with  the  temperature  up  to  an  optimum 
point,  beyond  which  it  declines  until  an  upper  limit  is 
reached,  which  again  stops  germination  entirely.  The 
lower  limiting  or  minimum  temperatures  for  germination 


i]  AIR  REQUIRED  FOR  GERMINATION  ii 

have  considerable  bearings  upon  practice;  gardeners 
know  well  that  certain  seeds  must  be  sown  "  in  heat "  or 
they  will  not  start ;  amongst  farm  crops,  the  seeds  of 
turnips  and  mangolds  make  very  little  progress  if  sown 
before  the  land  has  got  warmed  up  by  the  spring  suns 
and  cultivation. 

The  next  indispensable  factor  in  the  germination  of 
seeds  is  the  presence  of  air,  and  this  may  be  illustrated 
by  one  or  two  simple  experiments.  Put  some  mustard 
seeds  into  a  bottle,  fill  it  completely  with  water  and 
stopper  it,  then  let  it  stand  in  a  warm  place ;  a  few  of 
the  seeds  may  "  chit "  because  there  is  a  little  dissolved 
air  in  the  water,  but  they  will  stop  at  that  and  cease 
to  grow  any  further.  Grease  the  stopper  of  a  wide- 
mouthed  bottle  holding  about  a  pint,  put  in  half  an 
ounce  of  mustard  seeds  and  enough  water  to  wet  them 
thoroughly,  stopper  up  and  set  in  a  warm  place  as  before, 
but  this  time  in  the  dark.  In  a  few  days  the  seeds  will 
germinate,  and  as  there  is  some  air  in  the  bottle  they 
will  continue  to  push  for  a  few  days  before  they  come 
to  a  pause  and  stop  for  lack  of  more  air.  Now  see  what 
has  happened  to  the  air  inside  the  bottle  by  taking  out 
the  stopper  and  inserting  a  lighted  taper.  It  goes  out, 
indicating  that  the  oxygen  of  the  original  air  has  been 
so  far  used  up  that  a  candle  can  no  longer  burn.  Now 
decant  some  of  the  air  into  a  clean  bottle  and  shake  it 
up  with  lime  water :  the  lime  water  becomes  milky, 
indicating  the  presence  of  carbon  dioxide  or  carbonic 
acid.  The  germinating  seeds  have  thus  affected  the  air 
with  which  they  were  enclosed,  just  as  a  burning  candle 
would  do  or  a  breathing  animal ;  oxygen  has  been  used 
up,  and  carbon  dioxide  has  taken  its  place ;  something, 
in  fact,  in  the  seed  has  been  burnt,  i.e.,  combined  with 
the  oxygen  of  the  air  to  form  carbon  dioxide  and  water. 
The  identity  of  the  process  going  on  during  germination 


12  WHAT  THE  PLANT  IS  MADE  OF        [chap. 

with  burning  and  breathing  may  be  further  demonstrated 
by  showing  that  heat  is  given  off  during  the  process. 
To  prove  this,  a  pair  of  sensitive  thermometers  are 
necessary,  reading  to  a  tenth  of  a  degree ;  put  a  mark 
on  each  and  let  them  stand  in  the  same  jar  of  water  for 
a  time  to  note  any  difference  in  their  readings.  Mean- 
time, soak  about  a  pint  of  peas,  or  mustard,  or  barley  in 
water  for  a  few  hours,  drain  off  the  water,  put  the  peas 
in  a  jar  and  plunge  the  thermometer  into  their  midst. 
Fill  up  another  jar  with  water,  in  which  insert  the 
second  thermometer,  wrap  both  jars  in  flannel  to  reduce 
losses  of  heat,  and  then  stand  the  pair  in  some  place 
where  they  are  screened  from  any  but  slow  changes  of 
temperature,  as  in  an  inner  cupboard  of  a  room  with- 
out a  fire.  In  a  day  or  two,  when  germination  has 
set  in  actively,  the  thermometer  among  the  seeds  will  be 
standing  permanently  higher  than  the  other  one  which 
indicates  the  normal  surrounding  temperature.  With 
this  further  proof  we  may  conclude  that  germination  is 
a  process  of  breathing  or  burning,  in  which  some  of  the 
seed  material  combines  with  the  oxygen  of  the  air  to 
produce  carbon  dioxide  and  water ;  only  by  this  sort  of 
burning  does  the  infant  plant  get  the  energy  to  go  on 
living  and  working.  Indeed,  even  while  the  seed  is  dry 
and  apparently  dormant  it  is  breathing  very  slowly,  and 
so  consuming  part  of  its  substance;  being  dry,  the 
embryo  cannot  draw  upon  the  materials  in  the  endosperm, 
but  is  confined  to  burning  up  any  spare  material  it 
possesses  within  itself.  Hence  seeds  cannot  live  for 
ever  ;  they  vary  very  greatly  in  their  powers  of  endurance, 
both  with  the  kind  of  seed  and  the  way  they  are  stored, 
but  very  few  of  the  ordinary  farm  and  garden  seeds 
remain  alive  after  ten  years.  The  embryo  is  found  to 
perish  and  shrivel  up,  though  the  endosperm  or  coty- 
ledons  remain  perfectly  sound ;  the  slow  combustion 


I.]  VITALITY  OF  SEEDS  13 

process  necessary  to  maintain  life  exhausts  all  the 
available  material  in  the  embryo,  which  then  perishes, 
but  long  before  death  the  vigour  of  the  seed  becomes 
greatly  weakened  by  age.  There  seems  some  evidence 
that  seeds  buried  in  the  ground  may  retain  their  vitality 
for  much  longer  periods  than  seeds  stored  in  the  open 
air,  perhaps  because  the  greater  supply  of  moisture 
enables  the  embryo  to  draw  upon  the  food  store  to 
support  the  combustion  process,  instead  of  dying  as  soon 
as  it  has  used  up  all  its  own  stock  ;  but  the  evidence  that 
long-buried  seeds  do  retain  their  vitality  is  still  a  little 
doubtful.  Of  course,  germination  of  the  so-called 
"  mummy  wheat "  is  a  fairy  tale  ;  whenever  wheat  grains 
are  found  that  were  indubitably  buried  with  the  mummy, 
not  only  has  the  embryo  perished  but  the  material  of 
the  endosperm  is  carbonised  just  as  if  it  had  been 
charred,  so  that  it  is  no  longer  capable  of  forming  food 
for  an  embryo. 

Since  the  food  store  of  the  cotyledons  or  endosperm 
exists  for  the  purpose  of  maintaining  the  embryo  until  it 
has  built  up  a  plant  capable  of  carrying  on  an  independ- 
ent existence,  it  is  clear  that  if  this  latter  process  is  too 
long  deferred,  the  food  store  may  give  out  and  the  plant 
perish.  The  larger  the  seed  the  more  reserve  there  is, 
and  the  longer  the  young  plant  can  grow  on  the  seed 
store  alone ;  for  this  reason  the  depth  at  which  seeds 
can  be  sown  is  determined  by  their  size.  Provided  it 
does  not  get  so  far  down  as  to  be  cut  off  from  the  daily 
warmth  of  the  sun  or  from  the  air,  the  deeper  a  seed  is 
sown  the  better,  because  it  is  thereby  more  sure  of 
obtaining  a  continuous  supply  of  moisture.  But  minute 
seeds,  like  those  of  a  poppy,  do  not  possess  enough 
material  to  give  rise  to  more  than  a  very  short  stem  or 
leaf  with  which  to  reach  up  to  the  light :  mustard  seeds 
can  hardly  struggle  up  3  inches,  while  6  inches  taxes 


14  WHAT  THE  PLANT  IS  MADE  OF        [chap. 

all  the  resources  of  wheat  and  barley.  A  few  simple 
experiments  with  seeds  of  various  sizes,  sown  at  different 
depths  in  pots,  will  show  how  the  proper  depth  for  each 
seed  is  conditioned  by  its  weight,  i.e.  by  the  amount  of 
material  it  has  available  to  lift  a  leaf  up  into  the  light. 
Some  covering  of  soil  is  desirable  in  order  to  keep  the 
seed  from  drying  out ;  as  far  as  the  light  goes  it  does 
not  much  matter,  for  all  our  common  seeds  germinate 
equally  well  in  light  or  darkness. 

One  other  point  in  the  germination  of  seeds  requires 
consideration :  the  materials  in  the  food  store  of  the 
endosperm  or  cotyledons  are  mostly  insoluble,  as  we 
can  convince  ourselves  by  a  little  experiment ;  they 
consist  as  a  rule  of  starch  and  proteins,  with  sometimes 
fat  and  oil.  Yet  when  the  embryo  begins  to  grow  these 
materials  have  to  be  conveyed  to  the  end  of  the  shoot 
where  leaves  are  unfolding,  and  to  the  growing  tip  of 
the  root ;  that  they  do  get  transferred  is  proved  by  the 
eventual  emptying  of  the  endosperm  or  cotyledon. 
The  transference  is  only  possible  if  the  food  store  is 
first  of  all  rendered  soluble  in  water,  for  liquids  alone 
can  traverse  the  cells  of  the  plant.  The  mechanism  of 
the  process  can  only  be  rendered  intelligible  by  another 
experiment  or  two.  Soak  a  handful  of  barley  in  water 
as  before,  and  put  it  aside  in  a  warm  place  to  germinate 
until  the  rootlets  are  an  inch  or  so  long ;  now  smash  the 
grains  up  in  a  mortar,  add  enough  warm  water 
(at  40°  to  50°  C.)  to  make  up  a  sort  of  thin  porridge,  and 
let  the  whole  stand  for  an  hour  or  two,  then  filter. 
Meantime  make  some  starch  paste  by  rubbing  up  a 
few  grammes  of  starch  with  cold  water,  and  pouring 
on  300  to  500  c.c.  of  boiling  water;  then  bring  the 
whole  up  to  a  boil,  and  put  it  aside  to  cool.  The  starch 
paste  will  form  a  thick,  troubled-looking  liquid,  and  if 
to  a  little  of  it  in  a  test-tube  a  drop  of  a  solution  of 


I.]  A  CTION  OF  ENZ  YMES  1 5 

iodine  in  alcohol  or  potassium  iodide  solution  be  added, 
an  intense  blue  colour  will  be  produced :  this  is  a  test 
from  starch  that  we  shall  have  to  use  repeatedly. 

When  the  starch  paste  has  cooled  down  to  50^  C.  or 
so,  and  the  germinated  barley  extract  has  been  filtered 
clear,  pour  about  100  c.c.  of  starch  paste  into  a  beaker 
and  add  to  it  10  c.c.  of  the  barley  extract,  stirring  them 
well  together.  Keep  the  mixture  on  a  warm  bath  at 
50°  to  60°  C,  stir,  and  watch  the  result :  almost  instantly 
the  starch  paste  will  begin  to  get  thin  and  limpid,  and 
if  at  intervals  a  little  is  taken  out  and  tested  with  iodine 
the  blue  colour  will  begin  to  give  place  to  purple,  and 
will  finally  cease  to  appear.  When  this  is  the  case, 
taste  the  mixture  ;  it  will  be  faintly  sweet,  and  on  testing 
it  will  be  found  to  contain  sugar.  About  one-quarter 
fill  a  test-tube  with  Fehling's  solution,  add  half  as  much 
of  the  converted  starch  paste,  and  bring  to  the  boil : 
the  blue  colour  disappears,  and  there  is  a  copious 
precipitate  of  bright  red  oxide  of  copper.  This  is  a 
test  from  sugars  we  shall  often  have  occasion  to  use ; 
if  it  is  tried  on  the  unconverted  starch  paste,  no  change 
takes  place.  Clearly,  then,  we  have  extracted  from  the 
germinated  barley  some  soluble  substance  which  is 
capable  of  transforming  the  thick  starch  paste  into 
freely  soluble  sugar ;  it  will  do  the  same  to  solid  starch 
grains,  as  may  be  seen  under  the  microscope.  Dab  a 
tiny  bit  of  cut  potato  on  a  glass  slip,  the  little  smear 
it  leaves  consists  of  starch  grains,  flood  them  with  a  drop 
of  the  barley  extract,  put  on  a  cover  slip,  and  place 
under  the  microscope.  In  a  few  hours  the  grains  will 
begin  to  get  etched,  and  will  finally  break  up  and 
disappear.  Now  repeat  the  first  experiment  with  the 
starch  paste,  but  this  time  boil  the  barley  extract  before 
adding  it — no  change  to  sugar  will  take  place ;  the  boil- 
ing has  destroyed  the  substance  in  the  barley  extract 


i6  WHAT  THE  PLANT  IS  MADE  OF        [chae 

which  can  convert  starch  into  sugar.  This  substance  in 
the  extract  of  the  germinated  barley  belongs  to  the 
class  of  bodies  known  as  enzymes,  or  soluble  ferments  ;  it 
is  called  diastase,  and  it  is  secreted  by  the  active  growing 
embryo  of  the  barley,  its  function  being  to  convert  the 
stored-up  starch  of  the  endosperm  into  soluble  sugar 
which  can  travel  to  the  growing  points  of  the  plant. 
There  are  many  kinds  of  enzymes  secreted  by  both 
animals  and  plants ;  they  are  mostly  soluble  in  water ; 
they  all  work  most  rapidly  at  a  temperature  of  50°  to 
60°  C.  (122°  to  140°  F.),  and  are  destroyed  at  or  below  the 
temperature  of  boiling  water.  By  very  similar  experi- 
ments we  can  show  that  the  germinating  seed  also 
contains  a  protease,  i.e.  an  enzyme  attacking  the 
insoluble  proteins  and  converting  them  into  soluble 
amino  and  amido  bodies.  Pepsin  and  papain  are 
commercial  proteases,  extracted  one  from  an  animal  the 
other  from  a  plant,  and  it  is  easy  to  show  that  they  will 
dissolve  insoluble  proteins  like  fragments  of  boiled 
white  of  Qgg  or  gluten.  Similarly,  the  embryos  of 
germinating  seeds  of  mustard  or  flax  secrete  a  lipase, 
or  fat-splitting  enzyme,  capable  of  reducing  fat  to  soluble 
substances  which  can  traverse  the  plant  and  be  utilised 
at  the  growing  points. 

The  germinated  barley  from  which  we  have  just  been 
extracting  diastase  is  nothing  but  incipient  malt;  the 
process  of  malting  consists  in  steeping  good  well-ripened 
barley  for  forty-eight  hours,  and  then  spreading  it  in  a 
layer  4  to  12  inches  thick  on  floors  to  germinate.  After 
ten  or  eleven  days,  when  the  rootlets  are  almost  twice  as 
long  as  the  grain,  the  barley  is  slowly  and  carefully 
dried  at  a  temperature  not  exceeding  i8o°F.  Finally 
it  is  screened  to  knock  off  the  rootlets,  and  put  into 
store.  The  germination  process  develops  a  quantity  of 
diastase  which  accumulates  in  the  embryo  and  is  not 


I.]  SEED  TESTING  ij 

destroyed  at  the  temperature  of  drying,  so  that  when  the 
malt  is  afterwards  ground  and  extracted  with  water — the 
mashing  process — there  is  sufficient  diastase  to  convert 
quickly  all  the  starch  of  the  endosperm  into  sugar,  which 
is  afterwards  fermented  by  the  yeast  into  alcohol. 

Having  thus  considered  the  factors  that  determine 
the  germination  of  seeds,  we  are  in  a  position  to  consider 
the  application  of  them  to  practice.  In  the  first  place, 
the  seed  must  be  alive  ;  it  must  not  have  begun  to 
sprout  and  then  been  dried  off  again  ;  it  must  not  have 
been  overheated  in  the  stack,  or  harvested  prematurely 
before  the  embryo  had  come  to  its  full  term ;  it  should 
not  be  so  old  as  to  have  declined  in  vitality.  It  is  easy 
to  make  a  germination  test  of  the  common  farm  or 
garden  seeds;  select  a  soft  tile,  and  with  a  file  score 
ten  parallel  grooves  across  it,  then  ten  more  crossing 
them  at  right  angles,  thus  giving  rise  to  lOO  points  of 
intersection.  Stand  the  tile  in  a  dish  of  water  so  that 
the  scored  surface  is  above  the  water  but  is  kept  damp 
by  the  porous  nature  of  the  tile,  and  set  out  lOO  of  the 
seeds  taken  at  random,  one  on  each  intersection.  Cover 
over  with  another  plate  or  dish,  set  it  in  a  warm  place 
and  examine  every  day,  counting  the  number  that  have 
started,  and  removing  them.  A  saucer  of  fine  sand  or 
even  three  or  four  thicknesses  of  blotting  paper  will  do 
as  well  as  the  tile,  though  the  water  supply  may  require 
a  little  more  attention.  Most  farm  seeds  will  have  ger- 
minated in  ten  days,  but  mangolds  require  a  fortnight, 
and  the  majority  of  grass  seeds  three  weeks.  If  the 
records  are  properly  kept,  a  table  is  finally  obtained  which 
shows  both  the  percentage  which  eventually  germinate 
and  the  rapidity  with  which  they  start;  with  a  good 
young  vigorous  sample,  most  of  the  seeds  start  within 
a  day  or  two  of  one  another  and  comparatively  early. 

Given  good  seed,  it  is  of  no  use  to  sow  it  until  the 

B 


i8  WHAT  THE  PLANT  IS  MADE  OF     [chap.  i. 

year  is  advanced  enough  to  ensure  sufficient  warmth 
in  the  soil,  or  without  a  proper  preparation  of  the  land. 
Of  course  some  seeds  require  artificial  heat,  but  all  will 
grow  more  quickly  on  a  southern  slope,  or  where  the  soil 
is  not  kept  cold  by  lack  of  drainage  or  insufficient 
preparation.  The  great  art  of  the  farmer — the  prime 
act  of  husbandry,  in  fact — lies  in  the  preparation  of  a 
proper  seed  bed,  and  its  character  may  in  most  cases  be 
summarised  in  two  words  :  fine  and  firm.  It  must  be 
fine,  to  ensure  that  all  the  seed  can  be  put  in  at  the 
proper  depth,  because  there  is  a  proper  depth  for  each 
seed  depending  on  its  size ;  it  must  be  firm,  to  keep  the 
seed  and  the  infant  plants  properly  supplied  with 
moisture.  Sometimes  the  seed  is  soaked  before  sowing, 
though  on  the  farm  this  is  only  occasionally  done  with 
mangold  seed,  which  has  a  thick,  coarse  husk.  Occasion- 
ally the  soaking  may  very  much  quicken  up  the 
germination,  but  it  is  not  so  easy  to  sow  wet  or  even 
damp  seed  evenly,  and  there  is  some  danger  in  sowing 
soaked  seed  in  dry  soil,  lest  a  drought  should  follow  and 
the  seed  should  perish  after  its  premature  start. 
Lastly,  as  air  is  necessary  to  the  germinating  seed,  the 
soil  must  be  working  freely  at  sowing  time ;  if  the  seed 
is  plastered  into  wet  heavy  soil  or  the  surface  is  smeared 
over  by  incautious  cultivation,  the  seed  may  die,  or 
become  very  weakly  for  want  of  air.  Certain  kinds  of 
heavy  land,  again,  are  apt  to  develop  a  tight,  glazed  skin 
on  the  surface,  if  seeding  on  a  fine  tilth  is  followed  by 
heavy  rain  and  later  by  drying  winds  and  sun  ;  a  roller 
should  be  put  over  the  land  to  break  the  crust  and  let 
the  air  in  to  the  seed. 

As  soon  as  the  young  root  has  got  anchored  in  the 
soil  and  the  young  shoot  has  spread  its  first  leaves  into 
the  air,  the  function  of  the  seed  is  over  and  the  plant 
begins  its  independent  existence. 


CHAPTER   II 

THE  WORK  OF  THE  LEAF 

The  Increase  of  Weight  in  a  Growing  Plant  is  derived  from  the 
Air.  Plants  split  up  Carbon  Dioxide  and  give  off  Oxygen. 
Purification  of  the  Air  by  Plants.  Formation  of  Starch  in  the 
Green  Leaf.  Motive  Power  supplied  by  Light.  All  parts  of 
the  Living  Plant  are  also  breathing.  Necessity  of  Leaves  to 
the  Growth  and  Ripening  of  the  Plant.  The  Leaves  of  the 
Plant  give  off  Water.  How  much  Rainfall  is  required  by 
Crops. 

The  leaf  is  in  many  respects  the  most  important  part 
of  the  plant ;  it  is  essentially  the  manufacturing  organ, 
and  on  it  depends  the  whole  power  of  the  plant  to  grow 
— i.e.  the  fundamental  process  which  produces  fuel  and 
food  for  ourselves  and  the  rest  of  the  animal  creation. 

As  an  understanding  of  the  manner  in  which  a  leaf 
works  lies  at  the  very  foundation  of  our  science,  it  will 
be  necessary  to  consider  a  number  of  experiments  to 
illustrate  every  phase  of  the  action. 

In  the  first  experiment  weigh  out  a  few  grains  of 
mustard  seed,  and  if  you  have  not  previously  deter- 
mined what  percentage  of  water  the  mustard  seed 
contains,  weigh  out  a  second  portion,  and  put  this  to 
dry  in  the  oven.  Then  take  a  pot  filled  with  the 
purest  sand  and  mix  some  of  the  plant  ash  you  have 
previously  made  with  the  sand  at  the  top  of  the 
pot,   add   a   suitable   amount   of  water,   and   sow  the 

19 


20  THE  WORK  OF  THE  LEAF  [chap. 

mustard  seeds.  When  they  are  well  up  and  showing 
green  leaves,  add  one-tenth  of  a  gramme  of  nitrate  of 
soda  to  the  water  you  are  giving  to  the  pot,  and  let  the 
growth  continue  for  a  few  weeks  longer.  Finally,  when 
the  plants  show  signs  of  flowering  pull  them  up,  wash 
away  all  traces  of  sand  from  their  roots,  put  them  in  the 
oven,  and  weigh  them  when  dry.  They  will  weigh  eight 
to  ten  or  twenty  times  as  much  as  the  seed  from  which 
they  started.  It  is  not  even  necessary  to  add  to  the  pot 
of  sand  the  plant  ash  or  the  nitrate  of  soda — the  weighed 
seeds  may  be  sown  on  a  bed  of  sand  or  a  strip  of  clean 
flannel  kept  moist  in  a  soup  plate,  the  little  plants  being 
taken  off  when  they  will  grow  no  longer.  In  this  case, 
however,  the  increase  of  weight  will  not  be  so  great. 
Or,  again,  one  of  the  water  cultures  to  be  described 
later  may  be  used  to  afford  a  comparison  between  the 
weight  of  the  seed  and  the  weight  of  the  plant  arising 
from  it  when  it  has  not  been  in  contact  with  any  soil. 
Now  the  point  of  all  these  experiments  is  that  the 
increased  weight  of  the  plant  is  largely  made  of  carbon 
(we  have  already  seen  that  more  than  half  of  the  dry 
matter  of  a  plant  is  carbon),  yet  neither  the  sand,  the 
plant  ash,  the  nitrate  of  soda,  nor  the  water  used  in  the 
first  experiment,  nor  the  solution  used  in  the  water 
culture,  contain  any  carbon  at  all.  It  is  true  that  the 
flannel  we  have  also  suggested  as  a  medium  on  which 
the  mustard  seeds  could  be  grown  is  a  compound  of 
carbon,  but  it  is  such  a  compound  as  the  plant  cannot 
feed  upon,  and  is  as  inert  as  so  much  sand.  If,  then, 
the  plant  gains  carbon  as  it  grows,  and  there  is  none  in 
the  water  or  the  support"  on  which  it  grows,  where  does 
the  carbon  come  from  ?  There  remains  only  one  source 
— ^the  air  which  surrounds  the  plant ;  and  later  experi- 
ments will  make  more  evident  the  fact  that  plants  do 
derive  their  chief  sustenance  from   the  air.     In  these 


II.] 


CARBON  DRAWN  FROM  THE  AIR 


21 


preliminary  experiments  we  did  not  use  ordinary  soil, 
because  it  contains  a  certain  proportion  of  compounds 
of  carbon,  out  of  which  the  plant's  increase  might 
possibly  have  been  made,  though  we  can  now  show 
that  such  is  not  the  case.  Suppose,  instead  of  a 
laboratory  experiment,  we  deal  with  a  crop  in  the  open 
and  make  out  a  sort  of  balance-sheet ;  there  is  so  much 
carbon  in  the  soil  at  starting,  and  so  much  in  the  seed 
and  the  manure,  against  which  we  can  set  off  the 
amount  of  carbon  in  the  crop  at  the  finish  together 
with  that  in  the  soil  after  harvest.  Such  was  the 
balance-sheet  that  Boussingault  drew  up  for  the  first 
field  experiments  that  were  ever  carried  out,  and  he 
showed  that,  despite  the  great  amount  of  carbon 
removed  in  a  crop,  the  soil  contained  no  less  of  this 
substance  at  harvest  than  at  seed  time. 

Hence  followed  the  inevitable  conclusion  that  since 
the  carbon  had  not  come  from  the  soil,  and  could  not 
have  been  derived  from  the  rain  (water  contains 
hydrogen  and  oxygen  only),  it  must  have  been  taken 
from  the  air.  We  can  take  an  example  of  this  kind  of 
balance-sheet  from  the  Rothamsted  wheat  field,  choosing 
a  plot  manured  only  with  inorganic  salts  containing  no 
carbon  (superphosphate,  sulphate  and  chloride  of 
ammonia,  sulphate  of  potash,  etc.),  the  soil  of  which 
had  been  analysed  in  1881  and  again  in  1893. 


Table  III.— Carbon  in  Soil  and  Crops,  Rothamsted  Wheat. 
(Plot  7.) 


Carbon  in  Soil,  lb.  per  Acre. 

Crops. 
Average. 

Total 
Carbon 
in  Crop. 

Carbon 
in  Seed. 

Gain  in  Crop 
over  Seed 
and  Loss 
from  Soil. 

1881. 

1898. 

Gain  or 
Loss. 

Grain. 

Straw. 

61,000 

54,000 

-7,000 

Bosh. 
33-5 

Cwt. 
31-8 

25,200 

600 

17,600 

22  THE  WORK  OF  THE  LEAF  [chap. 

Thus,  though  the  carbon  in  the  soil  has  only  lost 
7000  lb.  during  the  period  under  examination,  no 
less  than  25,200  lb.  per  acre  has  been  taken  away  in 
the  crops,  and  the  air  is  the  only  source  from  which  it 
could  have  been  derived.  The  air,  we  know,  contains 
from  3  to  4  volumes  of  carbon  dioxide  in  every  10,000, 
and  small  as  the  proportion  may  seem,  it  yet  represents 
an  enormous  quantity  of  carbon  dioxide  upon  which  the 
plant  can  draw. 

We  will  now  take  some  experiments  showing  that 
plants  have  the  power  to  split  up  this  carbon  dioxide  in 
the  air  and  take  the  carbon  from  it.  As  we  are  dealing 
with  gases  we  shall  have  to  put  our  experimental  plants 
in  water  in  order  to  see  and  collect  the  gases,  and  it 
will,  therefore,  be  necessary  to  take  well  or  river  water 
that  has  had  an  opportunity  of  dissolving  a  little  air,  and 
especially  carbon  dioxide,  which  is  rather  more  soluble 
than  the  rest  of  the  air.  By  boiling  some  of  the  water 
in  a  preliminary  experiment  it  is  easy  to  show  that  such 
water  contains  dissolved  carbon  dioxide,  which  is  expelled 
in  heating.  Take  a  small  bunch  of  young  green  active 
shoots  of  mint  or  watercress  and  put  them  in  a  jar  of 
water,  covering  them  with  an  inverted  funnel  the  shank 
of  which  leads  into  a  test-tube  full  of  water.  Stand  the 
jar  in  the  brightest  light  available,  and  repeat  the 
experiment  with  another  jar  placed  in  the  dark.  To 
complete  the  demonstration  a  third  jar  should  be  used, 
containing  water  that  had  been  boiled  to  expel  all  gases 
and  then  cooled  down ;  this  also  should  be  placed  in  the 
light.  As  the  light  falls  on  the  mint  in  the  unboiled 
water  little  bubbles  of  gas  will  begin  to  appear  on  the 
tips  of  the  leaves ;  from  time  to  time  they  will  break 
away  and  ascend  into  the  test-tube  above.  No  such 
bubbles  appear  from  the  mint  in  the  dark,  or  from  that 
which   is  in  the  light  but  immersed  in  boiled  water. 


II.]  ox  YGEN  E  VOL  VED  B  V  PLANTS  23 

After  a  few  hours  of  bright  daylight  the  test-tube  will  be 
nearly  full ;  remove  it  carefully,  invert  it,  and  bring  into 
the  mouth  a  chip  with  a  glowing  end — the  chip  bursts 
into  a  flame,  proving  the  gas  to  be  oxygen.  Now 
oxygen  is  one  constituent  of  carbon  dioxide,  and  the 
experiment  is  an  illustration  of  the  fact  that  green 
leaves  in  the  light  split  up  the  carbon  dioxide  with 
which  they  are  in  contact  and  set  free  the  oxygen  as 
gas,  retaining  the  carbon  for  themselves.  This 
continual  evolution  of  oxygen  by  green  plants  is  the 
great  agency  which  is  always  renewing  the  vital  part  of 
the  atmosphere ;  out  in  the  country  where  the  trees  and 
the  grass  are  growing,  the  amount  of  carbon  dioxide  in 
the  air  keeps  at  about  3  volumes  in  10,000,  being  rather 
less  in  the  summer,  when  vegetation  is  active,  than  in 
the  winter ;  in  towns  the  proportion  rises  to  4,  and  even 
in  dense  streets  to  6  and  7  per  10,000.  On  plants 
growing  in  water  it  is  possible  to  see  the  bubbles  of 
oxygen  as  they  are  produced  ;  if  you  look  into  any 
ditch  or  pool  on  a  bright  day  you  will  see  the  bubbles  of 
oxygen  entangled  in  the  vegetation,  just  as  you  will  see 
them  dotted  all  over  the  green  algal  growth  which 
covers  the  bottom  of  many  streams.  The  surface  of 
some  streams  is  often  covered  with  little  masses  of 
floating  scum ;  this  scum  consists  of  the  algal  growth 
that  has  been  lifted  off  the  bottom  and  buoyed  up  to 
the  surface  by  the  bubbles  of  oxygen  entangled  in  it. 
It  will  be  noticed  that  the  scum  only  begins  to  float  up 
towards  the  afternoon  and  on  a  sunny  day,  i.e.  after  the 
light  has  had  time  to  act  and  bring  about  a  plentiful 
evolution  of  oxygen. 

So  far,  however,  we  have  only  demonstrated  that  the 
plant  adds  oxygen  to  the  atmosphere  ;  we  can  now  show 
that  it  will  remove  carbon  dioxide.  On  a  bright  day  in 
spring  or  early  summer  take  a  wide  glass  tube  5  or  6 


24  THE  WORK  OF  THE  LEAF  [cHAP. 

feet  long  and  fill  it  loosely  with  freshly  gathered  leaves 
of  grass  or  wheat,  taking  care  only  to  use  young,  active 
leaves.  Now  set  up  the  tube  horizontally  in  the 
brightest  daylight  available,  and  by  means  of  an 
aspirator  slowly  draw  air,  first  through  the  tube,  and 
then  through  a  wash-bottle  containing  some  baryta 
water.  Baryta  water  is  a  very  delicate  test  for  carbon 
dioxide,  becoming  troubled  and  milky  looking  with 
a  very  small  quantity  of  the  gas,  but  if  the  light  is 
good  there  will  be  no  evidence  of  carbon  dioxide  in 
the  air  that  has  been  drawn  through  the  tube.  Then 
detach  the  tube  and  let  the  ordinary  air  run  through 
the  baryta  water ;  there  is  at  once  a  milkiness,  showing 
what  must  have  been  the  state  of  the  air  before  it  came 
in  contact  with  the  green  leaves.  This  experiment 
illustrates  the  fact  that  green  leaves  in  bright  light  take 
the  carbon  dioxide  out  of  the  air,  and,  as  we  have  seen 
in  the  previous  experiment,  liberate  the  oxygen  con- 
tained therein. 

By  using  the  same  apparatus  with  petals,  bracts,  and 
other  parts  of  a  plant  which  are  not  green,  it  can  be 
shown  that  only  the  green  portions  of  the  plant  have 
this  power  of  absorbing  and  splitting  up  carbon  dioxide ; 
in  fact,  the  green  colouring  matter — the  chlorophyll,  as 
it  is  called — in  leaves  and  stems  effects  the  first 
necessary  step  in  the  process,  by  absorbing  the  light 
which  constitutes  the  motive  power.  That  light  is 
absolutely  essential  may  easily  be  seen  by  repeating 
the  last  experiment  after  covering  the  tube  containing 
the  leaves  with  brown  paper.  As  soon  as  this  has  been 
done  the  leaves  will  be  found  to  add  carbon  dioxide  to 
the  air  passing  through,  instead  of  taking  it  out ;  as  we 
shall  see  later,  all  parts  of  the  plant,  like  germinating 
seeds,  are  breathing  as  long  as  they  are  alive,  whereby 
they  give  off  carbon  dioxide  and  absorb  oxygen,  though 


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II.]       CARBON  DIOXIDE  SPLIT  UP  BY  PLANTS        25 

in  bright  light  this  breathing  process  is  masked  without 
being  stopped,  by  the  reverse  process  of  taking  in 
carbon  which  we  are  now  considering.  The  method  of 
experimenting  we  have  just  been  describing  has  been  so 
modified  as  to  become  a  means  of  measuring  exactly 
the  powers  of  the  leaf  in  splitting  up  carbon  dioxide 
under  various  regulated  conditions.  In  Dr  Horace 
Brown's  experiments,  a  leaf  still  attached  to  the  plant  is 
enclosed  in  a  glass-sided  box  and  a  current  of  air  is 
drawn  through  the  box,  the  proportion  of  carbon 
dioxide  being  determined  in  the  air  before  it  .reaches 
the  leaf  and  again  when  it  leaves  the  box.  Working  in 
this  fashion,  Dr  Brown  has  shown  that  a  square 
decimetre  (4  inches  square)  of  active  green  leaf  will 
decompose  about  o-oo8  gramme  per  hour  of  carbon 
dioxide  in  ordinary  diffuse  daylight,  which  amounts 
correspond  to  a  formation  within  the  leaf  of  about  o-oo6 
gramme  of  dry  matter.  The  light  may  be  diminished 
considerably  without  any  corresponding  falling  off  in 
the  power  of  the  leaf,  until  a  certain  point  is  reached 
beyond  which  the  action  will  fall  off  in  proportion  to  the 
reduction  in  the  light.  It  is  easy  to  see  how  dependent 
plants  are  upon  light  by  the  failure  of  most  of  them  to 
flourish  or  even  to  grow  in  anything  approaching  dense 
shade ;  only  a  few  classes  of  green  plants,  like  ferns  and 
mosses  and  those  generally  of  a  very  slow-growing 
character,  have  adapted  themselves  to  live  in  compara- 
tive shade,  whereas  all  farm  crops,  which  are  of  course 
distinguished  by  the  enormous  quantities  of  dry  matter 
they  manufacture,  must  be  exposed  to  the  fullest  possible 
light.  The  process  of  fixing  the  carbon  of  carbon 
dioxide  and  giving  off  oxygen  by  the  green  leaf  in  sun- 
light, which  we  shall  in  future  call  assimilation,  may  now 
be  studied  by  a  difTerent  method,  depending  upon  the 
fact  that  the  first  visible  product  of  the  action  in  the 


26  THE  WORK  OF  THE  LEAF  [chap. 

leaf  is  starch.  We  have  already  learnt  that  starch 
consists  of  the  element  carbon  combined  with  the 
elements  of  water,  from  which  it  follows  that  the  plants 
must  be  able  to  manufacture  starch  and  oxygen  out  of 
carbon  dioxide  and  water,  just  as  in  the  reverse  way 
starch  and  oxygen  will  burn  together  to  produce  water 
and  carbon  dioxide. 

Assimilation. 
Carbon  )  _    ,        j.     . ,  Carbon^  ^       ,      _ 

Oxygen  J  ^^^^°"^^°^*^®  +  ^^^^'^  =  Water  |  Starch  +  Oxygen. 

Of  course  the  process  of  making  starch  is  not  so  simple 
as  this  diagram  makes  out,  because  there  must  be  several 
intermediate  stages  within  the  plant,  still  for  our  pur- 
poses we  only  need  to  recognise  the  relationship  between 
the  final  products  of  the  change  in  order  to  interpret 
the  various  experiments  which  follow.  It  is  first  of  all 
necessary  to  learn  to  identify  starch  within  the  leaf; 
because  the  leaf  is  alive  and  strongly  coloured,  it  dis- 
guises the  blue  colour  which  iodine  usually  produces 
with  starch.  The  green  leaf  must  be  dipped  for  a 
moment  into  boiling  water  to  kill  it,  and  then  soaked  in 
alcohol  or  methylated  spirit,  until  the  green  colour  is 
dissolved  out.  If  now  the  bleached  leaf  is  dipped  into  a 
very  weak  solution  of  iodine,  the  colour  of  pale  sherry,  it 
will  be  turned  very  dark  blue,  practically  black,  because 
all  the  starch  grains  that  are  diffused  throughout  the 
leaf  are  intensely  stained  by  the  iodine.  The  leaf  can 
then  be  examined  under  the  microscope  to  see  how  the 
rounded  grains  of  starch  are  imbedded  in  all  the  cells 
composing  the  interior  tissues  of  the  leaf 

The  next  step  is  to  show  the  dependence  of  starch 
formation  upon  the  exposure  of  the  leaf  to  light ;  a 
dwarf  tropaeolum  plant  is  very  suitable  for  experiment, 
if  it  can  be  covered  completely  with  a  box  so  as  to 
exclude  the  light.     The  next  morning  but  one,  remove 


Fig.  5.— Artichoke  Leaf,  on  which  the  Light  Patch  was 

PROTECTED   FROM   LiGHT,  THE  StARCH   IN  THE  LEAF  BEING 
AFTERWARDS   STAINED   WITH    lODINE. 


[Face  page  26. 


II.]  STARCH  FORMATION  IN  THE  LEAF  27 

the  box  and  pick  off  one  or  two  leaves,  which  must  be 
put  away  in  a  box  until  they  can  be  brought  indoors  for 
testing ;  to  one  or  two  other  leaves  still  attached  to  the 
plant  fasten  opaque  designs  covering  part  of  the  leaf,  the 
simplest  plan  being  to  cut  two  discs  of  cork  and  pin  them 
together  with  the  leaf  between.     Leave  the  leaves  thus 
protected  in  places  from  the  light  until  evening,  and  then 
gather  them.     Follow  the  same  routine   as   before   in 
testing  the  leaves  for  starch ;  it  will  be  found  that  the 
leaves  which  have  been  kept  in  the  dark  for  thirty  to 
forty  hours  are  quite  devoid  of  starch,  from  which  we 
may  conclude  that  any  starch  present  in  the  leaf  when 
the  plant  was  covered  up  has  been  removed  in  the  dark. 
The  leaves,  however,  which  were   afterwards  partially 
exposed  to  the  light   show  a   pattern ;    the   protected 
portions  still  contain  no  starch,  whereas  the  rest  of  the 
leaf  on  which  the  light  has  fallen  has  gained  starch,  and 
so  will  colour  up  when  the  iodine  solution  is  applied. 
Further  experiments  depending  on  the  test  for  starch 
can  also  be  tried  ;  for  example,  a  variegated  leaf,  white 
and  green,  will  show  starch  in  the  green  parts  and  not 
in  the  white.     Again,  a  small  plant  may  be  kept  for  a 
day  or  two  under  a  bell-jar  over  water  with  some  strong 
cautic  soda  solution  standing  beside  it ;  the  caustic  soda 
will  remove  all  the  carbon  dioxide  from  the  air  with 
which  the  plant  is  in  contact,  and  the  leaves  after  a  day 
or  two  cease  to  contain  any  starch,  though  they  may 
have  been  in  full  sunlight  for  some  hours  before  they  are 
gathered.     In  this  way  it  is  possible  to  show  that  the 
formation  and  presence  of  starch  in  the  leaf  is  dependent 
upon  (i)  exposure  to  light ;  (2)  upon  the  greenness  of  the 
leaf;  and  (3)  upon  the  presence  of  carbon  dioxide  in  the 
air  with   which   the   leaf  is   in   contact ;    all   of  which 
conclusions  agree   with  the  views  of  assimilation  that 
we  had  before  obtained  by  other  methods. 


28  THE  WORK  OF  THE  LEAF  [chap. 

Though  starch  is  manufactured  in  the  leaf  it  does 
not  accumulate  there,  in  the  darkness  it  is  moved  wholly 
or  partially  away  and  stored  in  some  other  part  of  the 
plant :  were  it  not  so,  leaves  would  become  thick  and 
swell  with  the  stores  of  manufactured  starch.  The 
transference  of  the  starch  is  effected  by  a  similar 
mechanism  to  that  which  utilises  the  starch  in  a  seed 
for  the  use  of  the  distant-growing  parts  of  the  plant ; 
the  leaf  cells  secrete  an  enzyme  capable  of  dis- 
solving starch  and  transforming  it  into  sugar,  which 
being  soluble  in  water  can  move  about  in  the  plant. 
To  show  the  presence  of  this  diastase  we  must  gather 
some  young  and  active  green  leaves  in  the  early 
morning,  or  after  they  have  been  shut  up  from  the  light 
for  some  time,  as  in  a  previous  experiment.  Now  dry 
these  leaves  very  quickly  but  at  a  low  temperature,  best 
of  all  by  exposing  them  over  strong  sulphuric  acid  in  a 
desiccator  from  which  the  air  has  been  pumped.  Under 
these  conditions  the  leaves  will  dry  very  rapidly  and 
their  enzymes  will  suffer  no  change;  when  dry,  powder 
the  leaves  finely  and  introduce  a  little  of  the  powder  into 
a  starch  paste  solution  as  before.  The  starch  paste 
solution  will  rapidly  become  limpid,  will  lose  its  power 
of  turning  blue  with  iodine,  and  will  finally  give  a 
reaction  for  sugar  with  Fehling's  solution.  Clearly  the 
green  leaf  contained  an  enzyme  which  acts  on  the  starch 
paste  like  that  which  was  present  in  the  extract  from 
germinated  barley — a  diastase  transforming  starch  into 
soluble  sugar  capable  of  diffusing  out  of  the  leaf  into 
such  parts  of  the  plant  as  may  require  it.  So  funda- 
mental is  the  process  of  assimilation  that  yet  another 
experiment  may  be  described  to  illustrate  the  action. 
For  this  purpose  it  is  necessary  to  have  at  hand  a  plant 
or  plants  possessing  a  considerable  number  of  large 
leaves  :  the  vine,  hop,  or  sycamore  are  among  the  most 


II.]  STARCH  FORMATION  BY  THE  LEAF  29 

convenient  for  the  purpose.  Late  at  night,  when  the 
removal  of  starch  has  been  in  progress  for  some  time, 
about  three  hundred  leaves  with  their  stalks  attached 
should  be  gathered  and  brought  indoors.  From  the 
heap  of  leaves  pairs  of  approximately  equal  size  are 
picked  out,  and  as  each  pair  is  made  up  one  member  of 
it  is  placed  in  one  heap  which  we  will  call  A,  the  second 
on  another  heap,  B.  When  a  hundred  pairs  have  thus 
been  divided,  the  two  heaps  are  weighed  and  will  be 
found  approximately  equal,  because  the  errors  of  match- 
ing each  pair  of  leaves  will  neutralise  one  another  with 
so  large  a  number  as  a  hundred  pairs.  However,  there 
will  probably  be  some  small  difference  in  weight  which 
must  be  noted.  The  next  step  is  to  put  one  heap  (A)  of 
one  hundred  leaves  into  the  oven  to  dry,  the  other 
hundred  are  set  with  their  stalks  in  bottles  of  water  in  a 
position  which  will  expose  them  to  full  daylight  on  the 
following  morning.  At  the  close  of  that  day  they  also 
are  put  in  the  oven  to  dry.  On  comparing  these  sets  of 
dry  weights  (making  any  necessary  allowance  for  initial 
differences  of  green  weight)  the  leaves  that  have  been 
exposed  to  the  air  for  a  whole  day  will  be  found  to  have 
gained  in  weight,  because  of  the  starch  they  have  formed 
under  the  action  of  the  light 

We  have  already  described  one  experiment  to  show 
that  the  leaf  of  a  plant  is  also  carrying  on  a  second 
process,  one  of  respiration  or  breathing,  which  in  its 
result  is  exactly  opposite  to  assimilation,  in  that  it  burns 
up  some  of  its  carbonaceous  matter,  combining  it  with 
the  oxygen  of  the  air  by  which  it  is  surrounded  to  form 
carbon  dioxide  and  water.  It  is  possible  to  show  that 
each  part  ^of  the  plant  is  producing  carbon  dioxide, 
merely  by  gathering  such  parts  in  quantity,  shutting 
them  up  in  a  jar  for  an  hour  or  so,  and  then  testing  the 
gas  within  the  jar  with  lime  water  for  carbon  dioxide ; 


30  THE  WORK  OF  THE  LEAF  [chap. 

though,  of  course,  with  leaves  and  other  green  parts  the 
jar  must  be  shielded  from  the  light.  Respiration  is 
most  active  in  the  flower,  and  if  the  jar  is  filled  with  a 
lot  of  young,  active  tropseolum  flowers,  a  very  rapid 
evolution  of  carbon  dioxide  can  be  observed.  Respira- 
tion is  always  going  on,  though  it  is  reduced  to  a 
minimum  at  temperatures  below  40°  F. ;  it  is,  in  fact,  a 
process  essential  to  the  life  of  all  the  living  cells  that 
make  up  either  plants  or  animals.  In  a  leaf,  however, 
it  is  never  so  rapid  as  the  opposite  process  of 
assimilation  is  in  the  light ;  were  not  assimilation  very 
much  more  active  than  respiration,  the  plant  would 
never  increase  in  weight  at  all.  It  is  due  to  the 
respiration,  which  is  a  necessary  accompaniment  of  their 
life,  that  roots,  like  mangolds  or  potatoes,  gradually  lose 
weight  when  they  are  stored,  for  the  loss  is  not  only  of 
water  but  of  dry  matter  also.  Bulbs  of  tulips  or 
daffodils  are  also  noticeably  lighter  in  the  autumn 
when  they  are  planted  than  in  the  early  summer  when 
they  are  first  put  into  store,  and  if  the  planting  be 
deferred  for  any  reason  the  loss  of  weight  becomes  even 
more  evident. 

Another  experiment  may  be  carried  out  to  illustrate 
further  the  processes  of  respiration  and  assimilation : 
About  a  hundred  barley  grains  are  soaked  and  placed 
with  a  little  water  in  a  stoppered  bottle  holding  about  a 
pint,  the  stopper  is  left  out  and  the  bottle  exposed  to 
the  light  until  the  barley  has  shot  and  made  a  fair 
amount  of  leaf  The  stopper  is  then  inserted,  and  the 
bottle  is  put  away  in  a  dark  cupboard  or  drawer  for 
two  days ;  on  taking  it  out  and  testing  the  air  in  the 
bottle  by  means  of  a  lighted  taper,  the  taper  will 
be  extinguished.  In  the  dark  only  respiration  has  been 
going  on,  and  so  much  carbon  dioxide  has  been  pro- 
duced and  so  much  oxygen  used  up  that  the  air  in  the 


Fig.  6.— Bottle  enclosing  Barley  Seedlings. 

When  kept  in  the  dark  the  enclosed  air  puts  out  a  lighted  taper,  but  will  allow 
it  to  burn  after  the  bottle  has  been  for  some  hours  exposed  to  light. 


[Face  page  30. 


II.]  RESPIRATION  AND  ASSIMILATION  31 

bottle  is  no  longer  capable  of  supporting  combustion. 
Now  stopper-up  the  bottle  again  and  stand  it  in  the 
light  for  two  days ;  again  test  the  air  it  contains  with  a 
taper,  and  it  will  be  found  to  support  combustion.  In 
the  light  the  green  leaves  of  the  barley  have  split  up 
the  carbon  dioxide  and  replaced  it  by  oxygen,  until  the 
air  will  once  more  support  combustion.  The  bottle  may 
be  replaced  in  the  dark  until  respiration  has  again 
loaded  the  air  with  carbon  dioxide,  whereupon  it  may 
again  be  brought  into  the  light  and  the  air  reoxygenated 
by  the  assimilation  carried  out  by  the  green  leaves ; 
these  alterations  may  be  repeated  several  times,  until 
at  last  the  barley  begins  to  die  for  lack  of  food.  Under 
slightly  more  natural  conditions,  however,  such  a  cycle 
of  respiration  and  assimilation  can  be  continued  for 
very  long  periods.  For  example,  in  the  Rothamsted 
laboratory  a  sample  of  soil  weighing  several  pounds  was 
put  away  in  a  rather  moist  condition  in  1874  i^  3.  gallon 
bottle,  which  it  three  parts  filled.  The  bottle  was  corked, 
the  cork  waxed  and  covered  with  a  lead  capsule.  A  few 
years  afterwards  a  fern,  Asplenium  nigrum^  was  noticed 
to  be  growing  within  the  bottle,  having  developed  from 
a  spore  which  must  have  been  present  in  the  undried 
soil ;  this  fern  has  continued  to  grow,  and  at  the  present 
time  (19 10)  it  occupies  nearly  the  whole  of  the  vacant 
upper  part  of  the  bottle.  No  air  can  either  enter  or 
leave  the  bottle ;  at  first  some  doubts  were  entertained 
as  to  the  impermeability  of  the  sealed  cork,  but  the 
whole  of  the  cork  and  the  neck  of  the  bottle  have  been 
enclosed  in  a  block  of  paraffin  wax,  which  was  melted 
and  cast  on  to  its  present  position.  The  bottle  stands 
on  a  low  shelf  in  a  building  lighted  from  the  roof,  the 
illumination  it  receives  is  only  good  for  a  few  hours  on 
summer  days,  when  a  little  direct  sunshine  reaches  it. 
This  illumination  in  the  summer  is,  however,  enough  to 


32  THE  WORK  OF  THE  LEAF  [chap. 

enable  the  green  fronds  of  the  fern  to  decompose 
sufficient  carbon  dioxide  to  maintain  its  slow  rate  of 
growth,  and  also  to  give  rise  to  the  oxygen  required 
by  the  plant  for  respiration  throughout  the  whole  year. 
Similarly  the  plant  is  always  re-creating  carbon  dioxide 
by  its  respiration,  and  this  carbon  dioxide  must  be 
undergoing  a  perpetual  cycle  of  change — it  is  split  up 
by  the  plant  in  the  light  summer  days,  its  oxygen  being 
returned  to  the  air  and  the  carbon  held  by  the  plant, 
then  in  the  darkness  and  the  winter  this  change  is  more 
slowly  undone  again  and  the  carbon  dioxide  recon- 
stituted. Of  course,  in  such  a  fashion  the  fern  could 
never  become  any  heavier,  could  not  in  fact  have 
reached  its  present  size ;  there  has  also  been  another 
source  of  carbon  dioxide  within  the  bottle  due  to  the  slow 
decay  of  the  organic  matter  originally  present  in  the 
soil.  The  development  of  this  enclosed  fern  with  its 
quiet  annual  ebb  and  flow  might  be  looked  upon  as  a 
sort  of  perpetual  motion  machine,  but  we  must  not  fail 
to  recognise  the  fact  that  one  external  factor — the 
incidence  of  the  light — is  absolutely  necessary  to  the 
process.  It  is  this  light,  small  as  its  amount  may  seem 
to  be,  which  supplies  the  energy  required  to  drive  the 
machine,  and  this  fact  may  lead  us  now  to  consider 
the  assimilation  process  from  the  point  of  view  of  the 
exchanges  of  energy  which  go  on  during  the  life  of 
the  plant.  It  is  clear  that  a  plant  is  a  storehouse 
of  energy,  of  potential  work ;  given  enough  of  the 
plant — a  tree,  for  example — we  can  burn  it  under  a 
boiler  and  drive  an  engine  which  will  do  work  for  us. 
To  come  a  little  closer,  starch  will  burn  when  ignited 
and  supplied  with  air  or  oxygen ;  in  burning  it  will  give 
out  heat,  which  can  be  made  to  do  work.  Clearly,  then, 
starch  and  oxygen  contain  more  energy  than  the  carbon 
dioxide  and  water  into  which  they  are  turned  by  the 


i 


Fig.  7. — Fern  that  has  been  growing  inside  a  closed 
Bottle  for  Thirty-six  Years. 


[Face  page  32. 


II.]  ENERG  V  STORED  B  V  THE  PLANT  33 

burning   process.     In  consequence,  when   a  leaf  turns 
carbon  dioxide  and  water  into  starch  and  oxygen,  it  is, 
as   it   were,   pushing   something   uphill,   endowing   the 
resulting  product  with  more  energy  than  the  original 
material  possessed,  and  since  energy  cannot  be  created 
de   novo  any  more  than  matter  can,  this  energy  must 
have  been  obtained  from  somewhere  outside  the  leaf. 
The  energy  required  to  effect  the  change  is,  in  fact, 
obtained  from  the  light ;  the  green  leaf  simply  acts  as  a 
kind  of  transformer  of  the  energy  coming  from  the  sun 
in  the  form  of  light,  into  the  stored-up  energy  possessed 
by  vegetable  material.     Stephenson  was  absolutely  right 
when  he  called  coal  "bottled  sunlight,"  for  the  vitally 
important  feature  of  the  coal — its  power  to  burn  and 
give  out  heat,  whereby  it  becomes  a  source  of  work — is 
all  derived  from  the  light  which  fell  upon  those  early 
forests  in  which  grew  the  vegetation  that  nowadays  is 
preserved   as   coal.     The  plant  is  not  a  very  efficient 
transformer  of  the  energy  of  sunlight,  because  it  only 
picks  out  and  utilises  a  very  small  selection  from  the 
numerous   rays    making    up    the   light ;    according   to 
Dr  Horace  Brown's  researches,  the  leaf  only  succeeds 
in  utilising  for  the  manufacture  of  starch  about  Trnnr  of 
the  energy  of  sunlight  and  ^V  of  ordinary  diffuse  day- 
light.    Small  as  this  utilisation  of  the  sun's  energy  may 
seem  to   be,  we   must   not  fail  to  realise  that  in  the 
assimilation  of  the  green  leaf  we  see  at  work  the  only 
great  upbuilding  process,  by  which  energy  is  caught  and 
stored,  that  can  be  recognised  as  going  on  in  the  world. 
The  life  of  an  animal  is  a  down-grade  process  depend- 
ing upon  the  burning  up  by  respiration  of  materials 
which  have  been  previously  built  up  by  the  plant,  and 
though  the  animal  cannot  destroy  the  energy  that  was 
present  in   his  food,  he   transforms   it   into  low-grade 
forms  which    are   not   longer    utilisable.      Decay   and 

C 


34  THE  WORK  OF  THE  LEAF  [chap. 

burning  are  similarly  destructive  or  down-grade  pro- 
cesses, and  just  as  the  life  of  the  animal  is  absolutely 
dependent  on  the  preliminary  life  of  the  plant  to  build 
up  its  necessary  food,  so  for  our  engine  power  we  are 
largely  living  upon  irreplaceable  capital  stored  up  in 
the  coal  and  oil  that  were  manufactured  in  earlier  stages 
of  the  world's  history. 

The  fact  that  the  green  leaf  supplies  the  driving 
power  to  the  whole  machinery  of  the  plant  finds  a  good 
many  applications  in  practice.  It  explains,  for  example, 
how  it  is  possible  to  kill  out  perennial  weeds  like  thistles 
or  bearbine  by  persistent  hoeing,  which  does  not  give 
the  leaves  an  opportunity  of  working,  although  the  root 
keeps  throwing  up  fresh  shoots  from  its  latent  buds. 
Every  time  it  does  so,  some  of  the  material  stored  up  in 
the  root  is  used  to  make  the  fresh  shoot  and  get  it  up 
above  the  ground ;  if,  however,  the  new  leaves  thus  arising 
are  cut  off  before  they  have  had  time  to  manufacture 
any  surplus  material  and  send  it  down  to  the  root  for 
storage,  the  original  stock  is  somewhat  exhausted,  until 
by  a  repetition  of  the  process  the  plant  no  longer 
possesses  any  reserve  material  wherewith  to  lift  a  shoot 
above  ground.  Persistence  in  the  cutting-off  process  is, 
however,  necessary ;  it  is  no  good  to  spud  a  thistle  once 
during  the  year,  because  it  possesses  sufficient  reserve 
material  in  its  root  to  get  fresh  leaves  up  to  the  light  and 
they  soon  repair  the  losses. 

Again,  we  see  how  unwise  is  the  practice  of  many 
gardeners  who  remove  the  leaves  of  tomatoes,  vines,  etc., 
when  the  fruit  is  forming,  with  the  idea  of  throwing  all 
the  strength  of  the  plant  into  the  fruit  or  of  letting  in  the 
sun  to  ripen  the  plant.  The  removal  of  the  leaves  means 
that  the  manufacturing  processes  of  the  plant  are 
stopped;  no  more  carbohydrates  are  formed,  in  these 
cases  no  more  sugar,  and  the  fruits  remain  small  and 


Fig.  8.— Transpiration  of  Water  from  Leaf. 


[Face  page  35. 


II.]  LEAVES  AND  RIPENING  35 

without  their  proper  sweetness  because  of  the  cutting- 
ofif  of  the  supply  of  sugar.  Stopping  the  growing  points 
of  the  plant  is  justifiable,  because  thereby  the  material 
manufactured  together  with  whatever  may  be  in  reserve 
in  the  stems  or  roots  will  be  thrown  wholly  into  the  fruit 
and  is  not  wasted  in  making  unnecessary  new  growth  ; 
but  to  the  final  heaping  up  of  sugar  and  similar  sub- 
stances in  the  fruit  the  continued  action  of  the  leaf  is 
indispensable.  In  the.  case  of  grapes,  it  has  been  shown 
that  to  cut  off  any  large  proportion  of  the  leaves  with 
the  idea  of  more  completely  exposing  the  grapes  to  the 
sun  reduces  not  only  the  size  of  the  grapes,  but  their 
richness  in  sugar  as  well.  What  ripening  action  is 
caused  by  the  direct  rays  of  the  sun  is  not  known,  but 
probably  the  increased  warmth  they  cause  induces  in 
apples  and  pears  a  quicker  change  of  starch  and  similar 
materials  into  the  sugar  and  aromatic  bodies  which  mark 
the  ripe  fruit.  The  sunny  side  of  the  fruit  is  never, 
however,  very  much  ahead  of  the  shaded  side,  so  that  we 
may  conclude  that  the  shading  is  not  of  much  moment 
in  delaying  ripening,  whereas  the  absence  of  leaves  has 
a  seriously  detrimental  effect. 

Assimilation  and  respiration  do  not,  however,  represent 
the  whole  of  the  work  of  the  leaf;  it  has  one  other 
fundamental  piece  of  work  to  do,  that  is,  to  get  rid  of 
water  from  the  plant.  We  can  illustrate  this  action  by 
several  simple  experiments ;  for  the  first,  take  a  test-tube 
with  a  well-fitting  soft  cork,  and  slit  the  cork  in  two  down 
its  length.  Then  introduce  a  long  thin  leaf  (a  barley  or 
a  daffodil  leaf  will  do,  according  to  the  time  of  year)  into 
the  tube,  and  cork  up  the  tube  with  the  leaf  between  the 
two  halves  of  the  cork.  Leave  the  tube  thus  hanging  on 
to  the  plant  until  the  next  day,  when  a  plentiful  deposit 
of  water  will  be  found  inside  the  tube.  For  a  second 
experiment,  take  a  pot  containing  some  actively  growing 


36  THE  WORK  OF  THE  LEAF  [chap. 

plant  possessing   only  a   single   stem,  a  geranium   for 
example,  and  tie  up  the   pot   in   waterproof  paper   or 
rubber  sheet  so  drawn  round  the  stem  of  the  plant  that 
no  water  can  evaporate  from  the  surface  of  the  pot  or 
the  soil  in  it.     Counterpoise  the  pot  on  a  large  balance, 
and  note  the  loss  of  weight  at  intervals  for  the  next  few 
days,  comparing  the  loss  per  hour  by  day  or  by  night, 
and  again  during  periods  of  varying  temperature,  etc. 
Instead  of  weighing  the  water  lost,  a  form  of  apparatus 
may  be  devised  to  render  visible  the  rate  at  which  the 
plant  is  giving  off  or  "  transpiring  "  water,  so  that  com- 
parative measurements  may  be  made  much  more  quickly. 
An  eight-ounce  bottle  is  provided  with  a  rubber  cork 
pierced  with  three  holes,  through  the  largest  of  which  a 
wooden  rod  passes,  while  a  second  carries  a  horizontal 
piece  of  capillary  tubing.     A  young  branch  of  a  plant 
or  the  stem  of  a  large  leaf  is  now  passed  through  the 
third  opening,  the  bottle  is  filled  with  water,  and  the  cork 
is  forced  in  so  that  no  bubbles  of  air  are  trapped  beneath 
the  cork.     When  the  apparatus  has  been  left  to  itself  for 
a  time,  until  it  has  become  adjusted  to  the  temperature 
of  the  room,  the  water  in  the  horizontal  tube  will  be 
seen  to  be  continually  receding  because  of  the  water 
taken  up  by  the  stem  and  transpired  from  the  leaves. 
By  means  of  a  scale  on  the  horizontal  tube  the  rate  at 
which  the  leaves  are  transpiring  may  be  measured,  and 
by  putting  the  apparatus  in  a  cooler  room,  by  shielding 
it  from  the  light,  etc.,  the  effect  of  various  factors  upon 
the  transpiration  process  may  be  gauged.     By  means  of 
the  wooden  rod,  the  water  in  the  bottle  may  be  forced 
back  into  the  capillary  tube  each  time  a  fresh  measure- 
ment is  required.     Measurements  should  not  be  made 
until  some  little  time  has  elapsed  after  each  change,  in 
order  that  the  plant  may  have  adjusted  itself  to  the  new 
condition.     In  this  way  it  can  be  shown  that  a  current  of 


Fig.  9.— Apparatus  for  Measuring  Rate  of  Transpiration 
FROM  Leaf. 


[Face  page  36. 


II.]  IVA  TER  GIVEN  OFF  B  V  LEA  VES  37 

air  on  the  leaves,  produced,  for  example,  by  a  bellows, 
increases  transpiration  if  it  does  not  lower  the  tempera- 
ture too  much,  whereas  a  close  saturated  atmosphere 
obtained  by  surrounding  the  leaves  with  a  bottle  and 
loosely  closing  the  mouth  will  reduce  and  almost  suspend 
the  process.  Syringing  the  leaves,  again,  by  inducing  a 
saturated  atmosphere  next  their  surface,  also  greatly 
reduces  transpiration.  Again,  by  smearing  with  vaseline 
first  the  upper  and  then  the  under  side  of  the  leaves,  we 
shall  find  that  with  most  plants  the  transpiration  takes 
place  almost  wholly  from  the  under  sides  of  the  leaves. 
We  can  demonstrate  this  fact  in  another  way  by  taking 
two  clean  but  cold  pieces  of  glass  and  putting  a  freshly 
gathered  leaf  between  them ;  on  taking  them  apart 
after  a  minute  or  two,  the  glass  will  be  so  dewed  over  as  to 
form  an  image  of  the  leaf,  but  the  image  will  be  very 
thin  and  faint  on  the  glass  which  was  in  contact  with 
the  upper  side  of  the  leaf  The  surfaces  of  a  leaf  should 
now  be  examined  under  a  microscope ;  either  the  leaf 
can  be  looked  at  without  any  preparation  by  reflected 
light,  or  the  skin  on  the  upper  and  the  under  sides  can 
be  torn  off  and  examined  separately  by  transmitted 
light.  The  surface  of  the  leaf  will  be  seen  to  be  studded 
with  small  mouth-like  openings,  called  stomata,  and 
these  openings,  except  with  a  few  plants,  are  very  much 
more  numerous  on  the  under  than  on  the  upper  side. 
The  stomata,  which  open  and  close  according  to  the 
illumination,  the  temperature,  the  degree  of  humidity  of 
the  air,  etc.,  form  a  means  of  communication  between 
the  outer  air  and  the  surface  of  the  active  cells  in  the 
middle  of  the  leaf;  indeed  the  cuticle  of  the  leaf,  except 
for  these  openings,  is  composed  of  cells  containing  no 
chlorophyll,  and  simply  acts  as  a  protecting  membrane 
practically  impervious  to  gases.  Through  the  stomata 
the  plant  takes  in  the  carbon  dioxide  it  requires  for 


38  THE  WORK  OF  THE  LEAF  [chap. 

assimilation  and  in  its  turn  exhales  oxygen;  through 
them  again  it  exhales  carbon  dioxide  when  it  is  respiring, 
and  gives  off  its  transpiration  water  in  the  form  of 
vapour.  As  a  rule,  the  water  escapes  as  vapour  and 
continues  to  be  invisible,  but  when  the  surrounding 
atmosphere  is  saturated  it  may  condense  as  drops  upon 
the  tips  or  edge  of  the  leaf  Thus  arises  the  greater 
part  of  the  dew  which  covers  the  grass  in  the  early 
morning ;  were  dew  only  water  that  had  been  condensed 
from  the  atmosphere  by  the  ground  cooled  by  radiation, 
dew  would  be  as  plentiful  on  a  garden  path  or  on  a 
stone  as  on  the  grass. 

Many  experiments  have  been  made  to  ascertain  how 
much  water  a  plant  transpires  during  its  growth,  and 
this  information  is  most  easily  applied  to  practical 
problems  if  we  establish  a  connection  between  the 
amount  of  water  transpired  by  a  plant  and  the  increase 
of  weight  (reckoned,  of  course,  as  dry  matter)  which  takes 
place  within  the  same  period.  At  bottom  there  is  no 
real  connection  between  assimilation  and  transpiration  ; 
the  two  processes  go  on  simultaneously,  and  to  some 
extent  are  similarly  affected  by  the  same  external 
conditions,  but  they  are  in  so  many  respects  inde- 
pendent that  any  ratio  we  may  trace  between  them  can 
only  be  a  sort  of  average,  true  for  the  general  conditions 
prevailing  in  the  place  of  experiment.  For  example,  in 
England,  Lawes  and  Gilbert  concluded  that  for  every 
pound  of  dry  matter  elaborated  by  such  plants  as  wheat, 
barley,  clover,  and  peas,  about  250  lb.  of  water  was 
evaporated  from  the  surface  of  the  leaves.  Hellriegel  in 
Germany  with  a  drier  atmosphere  obtained  results 
about  50  per  cent,  higher,  while  Wollny  in  Vienna  and 
King  in  Wisconsin,  with  still  hotter  and  drier  climates, 
obtained  even  higher  ratios.  In  England,  however,  we 
may  assume  that  every  pound  of  dry  matter  produced 


Fig.  10.— Stomata  of  Sweet  Pea,  Lower  Surface. 


[Fac^  page  38. 


II.]  WATER  REQUIRED  BY  CROPS  39 

by  a  crop  has  entailed  the  supply  of  from  250  to  300  lb. 
of  water,  and  this  mounts  up  to  a  considerable  quantity 
when  the  whole  production  per  acre  is  considered.  Of 
mangolds,  for  example,  we  may  expect  to  grow  with 
proper  treatment  40  tons  per  acre,  about  12  per  cent,  of 
which  will  be  dry  matter,  i.e.  4-8  tons  of  dry  matter  per 
acre  will  be  produced.  This  entails  the  evaporation  of 
4-8  X  250,  or  1200  tons  of  water  per  acre  ;  and  as  an  inch  of 
rain  is  approximately  equal  to  icx)  tons  of  water  per 
acre,  a  mangold  crop  of  40  tons  per  acre  must  evaporate 
through  its  leaves  as  much  as  1 2  inches  of  rain,  a  very 
considerable  proportion  of  the  annual  rainfall  in  the 
districts  in  which  mangolds  are  much  grown.  Other 
crops  do  not  produce  quite  as  much  dry  matter  per 
acre  as  the  mangold  does,  but  still  it  will  be  found  that 
most  of  our  farm  crops  necessitate  the  evaporation 
of  from  5  to  10  inches  of  rain.  Considering  that  crops 
make  their  chief  growth  during  the  hotter  periods  of  the 
year,  when  evaporation  from  the  soil  is  also  most  active, 
it  becomes  evident  why  the  amount  of  rainfall  is  one  of 
the  biggest  factors  in  crop  production,  and  why  the 
operations  of  the  farmer  in  cultivating  the  soil  are  very 
largely  directed  towards  conserving  for  the  plant  what- 
ever water  reaches  the  land. 

Because  of  the  large  amount  of  water  evaporated  by 
the  plant,  a  growing  crop  always  keeps  the  ground 
beneath  it  in  a  very  dry  condition ;  after  harvest  it  will 
generally  be  found  that  the  subsoil  is  dry  to  some 
considerable  depth.  For  example,  in  June  1870  at 
Rothamsted  after  a  long  drought,  determinations  were 
made  of  the  amount  of  water  in  the  soil  on  which  a 
barley  crop  was  being  grown,  and  on  an  adjacent  piece 
of  land  which  was  being  fallowed  and  kept  bare.  Down 
to  a  depth  of  54  inches  it  was  found  that  the  bare  fallow 
soil  contained  900  tons  per  acre  more  water,  an  amount 


40  THE  WORK  OF  THE  LEAF  [cHAP. 

equivalent  to  9  inches  of  rain.  It  is  often  impossible  to 
lay  drains  in  a  clay  soil  in  the  autumn  on  land  which 
has  been  cropped,  because  the  subsoil  is  so  dry  as  to  be 
unworkable  by  the  ordinary  drainage  tools.  Again,  the 
drying  action  of  the  crop  explains  the  difficulty  that  is 
often  met  with  in  the  south  and  east  of  England  in 
attempting  to  secure  a  second  catch  crop  by  sowing  on 
the  stubbles  immediately  after  harvest ;  unless  there  is 
timely  and  continued  rain,  both  soil  and  subsoil  are  too 
dry  to  support  any  satisfactory  growth.  Even  if  a  stand 
of  vetches  or  crimson  clover  is  established,  it  is  apt  to 
leave  the  land  in  spring  so  depleted  of  water  that  the 
succeeding  crop  of  roots  is  jeopardised  unless  there  is 
an  abundant  early  summer  rainfall.  Another  example 
of  the  drying  power  of  a  growing  plant  is  seen  in  the 
wide  circle  of  stunted  growth  which  extends  into  a  corn- 
field round  a  tree  in  a  hedgerow,  should  the  season  have 
been  a  dry  one.  By  its  roots  the  tree  has  robbed  the 
subsoil  of  water  much  more  than  it  has  deprived  the 
crop  of  either  light  or  nutriment.  The  fact  that  the 
growth  of  a  satisfactory  crop  necessitates  the  evapora- 
tion of  a  certain  amount  of  water  is  nowhere  more 
distinctly  recognised  than  in  the  practice  of  alternating 
one  or  two  years  of  crops  with  a  year  of  bare  fallow 
which  prevails  in  some  of  the  western  districts  of  North 
America,  and  also  in  Australia,  in  regions  where  there 
is  but  a  low  annual  rainfall  of  8  to  12  inches.  This 
amount  of  rain  is  insufficient  for  more  than  a  very  small 
crop  year  after  year ;  but  by  leaving  the  land  without 
crop  for  a  season  and  cultivating  the  surface  of  the  soil 
so  as  to  check  evaporation,  the  rainfall  of  the  fallow  year 
can  be  so  far  accumulated  that  when  combined  with  the 
normal  rainfall  of  the  following  year  a  profitable  crop 
can  be  grown.  Over  a  wide  area  of  semi-arid  country 
farming  can  be  made  profitable  without  irrigation  if  the 


II.]        DEVICES  TO  REDUCE  TRANSPIRATION         41 

rainfall  of  two  years  can  thus  be  stored  up  for  a  single 
crop,  or  sometimes  if  three  seasons*  rainfall  can  be 
utilised  by  two  crops  only. 

It  should  not  be  supposed  that  all  plants  demand  such 
great  quantities  of  water  for  transpiration  as  do  our 
ordinary  field  crops,  because  they  are  plants  which  have 
been  selected  for  their  rapid  growth  under  favourable 
conditions.  But  in  nature  we  always  find  that  plants 
which  have  adjusted  themselves  to  live  either  in  very 
dry  situations  or  in  places  where  it  would  be  injurious 
to  take  in  too  much  water,  have  always  modified  them- 
selves in  some  way  so  as  to  reduce  transpiration. 
Whereas  in  temperate  climates  the  leaves  of  a  plant 
arrange  themselves  to  wave  in  the  air  and  to  present  as 
great  a  surface  as  possible  to  the  light,  the  leaves  of  the 
Eucalyptus  and  most  of  the  other  Australian  trees  so 
dispose  themselves  as  to  present  only  an  edge  to  the 
sun,  thus  reducing  both  the  transpiration  and  the  heat- 
ing effect  of  its  rays.  Very  generally,  in  such  circum- 
stances the  plant  reduces  the  size  of  its  leaves  or  even 
replaces  them  entirely  by  spines,  gorse  or  whin  being  a 
case  in  point ;  sometimes,  as  in  the  stonecrops  and 
houseleeks  or  the  cacti,  the  leaf  is  made  very  thick,  so 
that  its  storage  capacity  is  great  as  compared  with 
its  evaporating  surface.  A  hairy  or  a  waxy  glaucous 
surface,  or  the  presence  of  resins  and  essential  oils 
(among  the  plants  growing  on  dry  banks  there  is  a 
notable  proportion  of  aromatic  ones),  are  all  regarded  as 
mechanisms  by  means  of  which  the  plant  has  learnt  to 
reduce  transpiration. 

Various  other  consequences  also  follow  from  the 
continued  loss  of  water  at  the  leaf  surface  by  transpira- 
tion, such  as  the  fliow  of  sap  in  the  stem  ;  but  first  it  will 
be  necessary  to  consider  at  some  length  how  the  water 
enters  the  plant. 


CHAPTER  III 

THE  WORK  OF  THE   ROOTS 

The  Roots  as  anchoring  the  Plant.  The  Roots  supply  the  Plant 
with  Water.  Roots  require  Air.  Roots  can  only  take  in 
dissolved  Material.  Etching  Action  of  Roots  due  to  their 
Excretion  of  Carbon  Dioxide.  Elements  necessary  to  the 
Nutrition  of  Plants.     Plants  require  Combined  Nitrogen. 

The  root  of  the  plant  has  two  great  functions  to 
perform — one  mechanical,  in  keeping  the  plant  firmly 
fixed  in  position ;  the  other  physiological,  in  supplying 
the  plant  with  water  and  the  food  it  requires  from  the 
soil.  In  certain  cases,  notably  the  mangold  and  the 
carrot  among  farm  plants,  the  root  is  also  utilised  as  a 
storehouse  of  the  reserve  material  that  is  being  accumu- 
lated in  readiness  for  the  formation  of  flowers  and  seed. 
The  books  devoted  specially  to  botany  must  be  con- 
sulted for  details  regarding  the  many  shapes  taken  by 
roots,  the  manner  in  which  they  grow,  and  the  devices, 
such  as  the  sensitive  root-tip,  which  they  possess  in 
order  to  make  their  way  into  the  ground  and  turn  in 
the  direction  of  food  and  water.  For  our  purpose,  the 
germinated  bean  and  wheat  seedlings  used  for  the 
previous  experiments  will  afiford  sufficient  information 
as  to  structure.  In  the  case  of  the  bean,  we  find  a  tap- 
root running  straight  down,  from  the  side  of  which 
secondary  roots  are  thrown  off;  it  may  further  be 
observed,  when  the  root  has  been  allowed  to  grow  into 
damp  air,  that  all  the  slender  roots  near  the  tip  are 

42 


CHAP.  III.]  ROOT  HAIRS  43 

clothed  with  a  down  of  very  fine  hairs,  though  the  last 
quarter  of  an  inch  or  so  is  bare.  In  the  case  of  the 
wheat,  instead  of  a  single  tap-root  a  number  of  fibrous 
roots,  which  branch  as  they  become  older,  issue  from 
the  embryo ;  these  likewise  are  clothed  with  root  hairs 
near  the  tips.  By  day-to-day  observations  we  shall  find 
that  the  root  hairs  fall  off  as  the  root  ages ;  they  occur 
only  in  a  region  just  behind  the  growing  point  of  the 
root.  By  carefully  digging  up  some  young  seedlings 
from  the  open  ground  or  from  a  pot  of  earth,  and  wash- 
ing away  the  soil,  we  shall  find  that  the  root  hairs  cling 
very  obstinately  to  some  of  the  fine  particles  of  soil,  with 
which  they  are  evidently  in  intimate  contact.  A  good 
deal  of  the  holding  power  of  roots  is  due  to  this  associa- 
tion of  the  root  hairs  with  the  finest  particles  of  the  soil. 
It  is  possible  to  show  by  appropriate  experiments  that 
the  primary  root  of  a  plant  tends  to  grow  straight  down- 
wards under  the  direction  of  the  pull  of  gravity,  but  that 
it  may  be  deflected  by  its  attraction  for  such  necessaries 
as  water,  air,  and  suitable  food.  The  attraction  exerted 
by  the  supply  of  air  is  plainly  to  be  seen  in  the  way 
roots  of  trees  and  any  deep-rooting  plants  force  their 
way  into  drains,  just  as  their  need  for  air  is  shown  by 
the  way  roots  will  not  penetrate  into  undrained  soil, 
but  stop  short  as  soon  as  the  layer  is  reached  which 
is  saturated  with  stagnant  water  and  contains  no  air. 
It  is  also  found  that  wheat  develops  the  best  root 
system  (the  foundation  of  a  large  crop  later)  when  the 
winter  is  comparatively  dry,  so  as  to  leave  the  soil  well 
aerated ;  in  a  wet  winter  or  wet  soil  the  plant  neither 
needs  so  extensive  a  root  system  in  order  to  keep 
itself  supplied  with  water,  nor  is  it  stimulated  by  the 
air  to  produce  roots.  The  attraction  exerted  by  food 
is  seen  in  the  way  a  bone  or  other  fragment  of  spar- 
ingly soluble  manure  becomes  covered  and  permeated 


44  THE  WORK  OF  THE  ROOTS  [chap. 

by  the  roots  of  any  crop  occupying  the  soil  in  which 
the  manure  is  buried. 

Of  the  power  of  roots  to  anchor  plants  and  hold  them 
firmly  in  position  little  demonstration  is  needed,  but  it 
is  interesting  to  see  how  many  plants  actually  pull  their 
crown  closer  into  the  ground  by  contracting  their  roots. 
Examine  the  young  dandelions  or  plantains  which  have 
established  themselves  on  a  lawn ;  not  only  is  there  no 
stem  lifting  the  crown  of  leaves  out  of  the  ground,  but 
on  the  contrary  this  crown  is  pressed  down  tightly  into 
the  ground.  As  a  matter  of  fact  it  has  been  pulled 
down  by  the  contraction  of  the  main  root ;  and  if  this 
main  root  is  carefully  dug  up  in  the  early  summer, 
washed  and  examined,  it  will  be  seen  to  be  corrugated 
and  to  possess  ridges  showing  where  the  contraction 
has  taken  place.  The  crocus  affords  another  interesting 
example  of  contractile  roots ;  each  year  the  new  crocus 
corm  is  formed  on  the  top  of  the  old  one,  so  that  after  a 
few  years  the  corm  would  find  itself  on  the  surface  of 
the  soil.  However,  the  new  corm  develops  a  ring  of 
fleshy  contractile  roots  which  draw  it  down  again  into 
the  position  occupied  by  the  old  one  before  its  decay. 
The  seedling  tubers  of  the  Common  Arum  (Cuckoo-pint, 
or  Lords-and-ladies)  possess  similar  contractile  roots 
which  operate  each  season  until  the  tuber  has  been 
drawn  down  to  its  proper  level.  The  habit  of  many 
plants  of  throwing  out  adventitious  roots  from  another 
part  of  the  stem  than  that  from  which  the  original  roots 
start  should  be  observed ;  in  the  case  of  cereals  this 
throwing  out  of  adventitious  roots  from  a  point  close  to 
the  surface  of  the  ground,  much  higher  up  than  the 
original  crown  of  roots  from  the  embryo,  is  a  very 
important  factor  in  supporting  the  straw;  like  the 
formation  of  side  shoots,  or  "  tillering,"  it  is  promoted 
by  free  exposure  to  light  and  air. 


Fig.  12. — Contractile  Taproot  of  Dandelion. 


[Fuce  page  44. 


III.]  WATER  CULTURES  45 

In  order  to  demonstrate  some  of  the  other  actions  of 
roots,  it  will  be  as  well  at  this  stage  to  raise  plants  in 
water  culture ;  barley  or  maize  form  convenient  plants, 
so  a  few  grains  should  be  germinated  in  damp  sawdust 
or  between  the  folds  of  a  roll  of  blotting  paper  standing 
vertically  with  the  bottom  just  dipping  into  water. 
Choose  some  twenty-ounce  bottles,  fit  the  necks  with 
corks,  and  make  up  a  solution  as  follows,  dissolving  the 
ingredients  separately  and  mixing  them  with  the  bulk 
of  the  liquid  in  the  order  given  : — 


Water    . 

I  litre. 

Potassium  Nitrate 

I  gramme. 

Magnesium  Sulphate    . 

•        0.5   „ 

Calcium  Sulphate 

.        0.5   „ 

Sodium  Chloride 

.        0.5   „ 

Monopotassium  Phosphate 

o-s   „ 

Ferric  Chloride 

trace. 

Wrap  the  outside  of  the  bottle  with  stout  brown 
paper,  and  tie  it  in  position  so  as  to  keep  the  contents 
in  the  dark.  When  the  seedlings  have  grown  far  enough 
to  show  a  little  shoot  and  roots  an  inch  or  more  long, 
split  the  corks  and  cut  a  notch  in  the  split  surface,  then 
fix  one  of  the  grains  in  the  notch  between  the  two  halves 
of  the  cork,  and  insert  them  into  the  bottle,  so  that  the 
tips  of  the  roots  just  dip  into  the  liquid.  If  the  notch  is 
too  large,  a  little  cotton-wool  may  be  necessary  to  fix  the 
grain  in  position.  Put  the  bottle  in  the  window  so  that 
the  little  plant  gets  full  light ;  if  water  is  added  to 
the  bottle  from  time  to  time  to  restore  what  has  been 
lost  by  evaporation,  the  barley  or  maize  plant  will  grow 
in  a  perfectly  normal  manner  and  will  ripen  seed  in  due 
course.  It  will  be  obvious  from  the  way  in  which  the 
water  has  to  be  constantly  renewed  in  the  bottle  when  the 
growth  is  active,  that  the  roots  of  a  plant  form  the  organs 
by  which  the  plant  takes  in  water,  a  fact  which  might 


46  THE  WORK  OF  THE  ROOTS  [chap. 

indeed  have  been  deduced  from  the  continual  loss  of 
water  by  transpiration  from  the  leaves.  By  weighing  a 
bottle  and  its  growing  plant  on  a  sensitive  balance,  and 
reweighing  after  a  given  time,  the  loss  of  water  by 
transpiration  under  different  conditions  of  temperature, 
illumination,  etc.,  can  be  readily  determined,  thus  check- 
ing the  conclusions  reached  by  the  previous  experiments 
upon  the  leaf  It  is  only  by  the  root  that  water  enters 
a  plant :  when  a  gardener  syringes  the  leaves  or  waters 
the  paths  and  stages  of  a  greenhouse  in  which  plants  are 
flagging,  he  does  not  thereby  add  water  to  the  leaf;  by 
saturating  the  air  with  moisture  he  checks  transpiration, 
and  thus  enables  the  intake  by  the  roots  to  make  up  for 
the  evaporation  at  the  leaf  surface.  Anything  that 
checks  the  development  of  root,  or  for  a  time  deprives 
the  plant  of  its  proper  amount  of  root,  renders  the  plant 
more  liable  to  die  from  lack  of  water;  thus  plants  after 
repotting  should  be  kept  for  a  short  time  in  a  close 
atmosphere  in  which  little  transpiration  can  take  place 
until  fresh  roots  have  developed,  and  a  transplanted  tree 
requires  special  attention  in  a  dry  season  to  maintain  a 
moist  soil  round  the  mutilated  root  system.  Again,  crops 
on  a  well-drained  soil,  even  though  it  is  light  and  little 
retentive  of  moisture,  will  withstand  a  period  of  drought 
longer  than  on  a  heavy  undrained  clay,  because  in  the 
former  case  the  root  system  extends  deeply  into  the  soil, 
whereas  in  the  latter  it  is  cut  short  near  the  surface  by 
the  stagnant,  airless  water.  A  plant  begins  to  wilt  and 
will  eventually  die  when  the  transpiration  from  the 
leaves  is  greater  than  the  supply  brought  in  by  the  root 
from  the  soil,  and  it  will  be  found  that  roots  are  not  able 
to  extract  the  whole  of  the  water  present  in  the  soil. 
If  plants  are  grown  in  a  pot,  and  as  soon  as  they  begin 
to  wilt  the  soil  is  turned  out  and  a  sample  weighed  and 
put  to  dry,  even  a  sandy  soil  will  be  found  to  contain 


III.]         DESTRUCTION  OF  PLANTS  BY  FROST  47 

about  3  per  cent.,  while  a  clay  may  contain  more  than 
10  per  cent,  of  water  which  the  soil  particles  can  hold 
against  the  plant,  so  that  it  therefore  is  useless  for  the 
support  of  vegetation.  That  the  roots  themselves  take 
water  from  the  soil,  and  do  not  merely  pass  on  the 
suction  exerted  by  the  transpiring  leaves,  may  be  seen 
by  the  manner  in  which  the  supply  of  water  to  the  plant 
ceases  when  any  such  cause  as  cold  stops  the  functioning 
of  the  roots.  The  vital  actions  of  the  root  are  suspended 
at  or  near  the  freezing  point,  and  water  ceases  to  be 
taken  up;  consequently  if  the  leaves  or  stem  of  the 
plant  are  subjected  to  any  great  drying  influence  while 
the  roots  are  thus  cold  and  out  of  action,  the  death  of 
the  plant  by  drying-out  may  result.  Indeed,  much  of 
the  destruction  by  frost  of  what  are  known  as  tender 
plants,  e.g.  tea  roses,  is  due  not  to  drought  so  much  as 
to  cold.  Such  destruction  will  always  be  found  most 
severe  if  a  spell  of  drying  wind  comes  when  the 
ground  is  frozen  and  there  is  no  snow  round  the  plants 
to  maintain  a  slightly  moist  atmosphere  and  prevent 
the  access  of  the  drying  wind.  The  shelter  of  a  little 
bracken  or  straw,  or  of  a  few  spruce  boughs,  is  sufficient 
to  preserve  the  plants  from  injury,  not  that  they  are 
thereby  maintained  at  any  higher  temperature,  but 
because  of  the  protection  from  wind  and  evaporation 
that  has  been  afforded.  The  morning  sunshine  falling 
upon  a  frozen  shrub  and  plant  is  most  destructive,  not 
because  the  sudden  thaw  can  exert  any  direct  harm, 
but  because  the  sun  starts  a  considerable  transpiration 
which  cannot  be  met  by  the  roots  in  the  still  frozen 
ground.  We  may  also  use  the  water  cultures  to 
demonstrate  the  fact  that  the  roots,  like  all  other  parts 
of  the  plant,  respire,  and  therefore  must  be  supplied  with 
oxygen,  in  place  of  which  they  give  off  carbon  dioxide. 
In  a  water  culture  the  roots  obtain  the  oxygen  they 


48  THE  WORK  OF  THE  ROOTS  [chap. 

require  from  the  small  quantity  that  the  water  holds  in 
solution  and  constantly  renews  from  the  atmosphere 
with  which  it  is  in  contact.  For  this  reason  it  is  advan- 
tageous to  blow  a  little  air  through  the  culture  solution 
once  a  week,  especially  when  the  root  development  has 
become  at  all  extensive.  The  result  of  cutting  off  the 
oxygen  can  be  shown  by  replacing  the  culture  solution 
in  one  of  the  bottles  by  some  water  which  has  been 
boiled,  to  free  it  from  any  dissolved  air,  and  then  cooled. 
Fill  up  the  bottle  with  this  water  close  to  the  neck  of  the 
plant,  and  then  pour  on  it  a  very  thin  layer  of  olive  oil, 
which  will  effectually  cut  off  the  access  of  air  to  the 
water  in  which  the  roots  are  distributed.  In  a  day  or 
so  the  plant  will  begin  to  show  signs  of  ill-health,  and 
will  rapidly  die.  That  the  respiration  of  the  root  is 
accompanied  by  the  evolution  of  carbon  dioxide,  can  be 
seen  by  placing  another  plant's  roots  in  distilled  water — 
not,  however,  freed  from  dissolved  air — pouring  off  this 
water  on  the  following  day,  and  adding  to  it  lime  water. 
The  milkiness  which  ensues  shows  that  carbon  dioxide 
has  been  given  off  from  the  root,  and  has  remained  to 
some  extent  dissolved  in  the  water.  The  same  demon- 
stration may  be  effected  by  letting  the  plant's  roots 
themselves  dip  into  lime  water. 

In  addition  to  the  water,  it  is  the  function  of  the  root 
to  take  in  the  various  substances  required  for  the 
nutrition  of  the  plant.  These  are  obtained  from  the  soil, 
and  constitute  the  plant's  ash.  A  simple  experiment 
can  be  carried  out  to  demonstrate  that  any  substances 
which  get  into  the  plant  must  first  of  all  be  dissolved  in 
the  water  in  contact  with  the  root.  Colour  the  solution 
in  one  of  the  water-culture  bottles  a  bright  pink  with 
eosin ;  to  another  bottle  add  carmine  or  Indian  ink.  The 
eosin,  being  in  solution,willgraduallypenetrate  the  young 
barley  plant,  and  may  be  seen  colouring  the  veins ;  but 


III.]  ETCHING  ACTION  OF  ROOTS  49 

the  carmine  and  Indian  ink  show  no  signs  of  penetration, 
because  in  their  case  the  colouring  matter  is  not  dis- 
solved, but  consists  of  very  fine  solid  particles  suspended 
in  the  fluid.  Flowers,  especially  those  from  bulbous 
plants,  which  readily  absorb  water,  are  occasionally  dyed 
by  thus  leaving  them  for  some  little  time  with  their  stems 
dipping  into  a  solution  of  some  dyestuff  which  makes  a 
true  solution. 

If  the  plant,  then,  can  only  take  in  materials  that 
have  been  first  dissolved  in  the  water  in  contact  with  the 
root,  how  comes  it  that  the  plant  can  make  any  use 
either  of  the  soil  or  of  a  great  number  of  manure 
substances  which  are  comparatively  insoluble  in  water  ? 
This  point  will  be  more  fully  dealt  with  when  we  are 
considering  the  soil,  but  at  this  stage  we  can  show  that 
the  plant  itself  helps  towards  bringing  such  substances 
into  solution.  For  the  experiment,  a  thin  slab  of 
polished  marble  will  be  wanted;  the  colour  is  of  no 
moment,  provided  that  the  surface  is  nicely  smooth  and 
bright.  This  slab  must  be  set  vertically  near  the  bottom 
of  a  pot  filled  with  ordinary  soil  in  which  two  or  three 
dwarf  beans  are  planted,  preferably  after  germination, 
in  order  to  save  time.  When  the  beans  have  grown 
pretty  well  and  the  pot  is  full  of  roots,  shake  out  the 
contents  and  wash  the  slab  free  from  all  dirt.  The 
polished  surface  will  be  found  to  be  etched  with  a  series 
of  markings  representing  the  places  where  it  has  been 
in  contact  with  the  roots,  which  thus  evidently  possess 
some  power  of  dissolving  the  carbonate  of  lime  com- 
posing the  marble.  From  this  experiment  and  the  fact 
that  the  sap  contained  in  the  roots  of  most  plants  is 
acid  to  litmus  paper,  it  has  been  somewhat  hastily  con- 
cluded that  the  plant's  roots  excrete  an  acid  sap,  or 
that  the  sap  acts  through  the  walls  of  the  root  and  so 
attacks  the  solid  particles  of  the  soil.    Probably,  however, 

D 


50  THE  WORK  OF  THE  ROOTS  [chap. 

this  view  is  erroneous ;  there  is  evidence  that  the  acid 
sap  can  never  reach  anything  outside  the  unbroken  root, 
and  all  the  actions,  including  the  etchings  of  the  marble, 
are  quite  explained  by  the  excretion  of  carbon  dioxide, 
which,  we  know,  is  always  taking  place  from  the  root. 
As  a  solution  of  carbon  dioxide  in  water  acts  like  a  weak 
acid  and  is  much  more  effective  than  pure  water  in 
dissolving  mineral  matter,  the  roots  of  the  plant  have  a 
distinct  effect  in  rendering  the  materials  in  the  soil 
available  as  food  for  the  plant ;  this  point  will  be 
considered  later  when  dealing  with  the  soil. 

We  may  now  proceed  to  make  use  of  the  method  of 
water  cultures,  to  determine  what  are  the  mineral  sub- 
stances, etc.,  taken  by  the  plant  from  the  soil,  and  which 
of  them  are  essential  to  its  growth.  When  dealing  with 
the  composition  of  the  plant,  we  have  already  seen  that  a 
comparatively  small  range  of  elements  are  to  be  found 
there,  and  that  all  plants  are  alike  in  containing  besides 
carbon,  hydrogen,  and  oxygen,  also  nitrogen,  sulphur, 
phosphorus,  chlorine,  and  often  silica,  with  potash, 
soda,  magnesia,  lime,  and  iron  among  bases.  Only 
these  elements  can  be  essential  to  the  plant :  and 
whether  all  of  them  are  necessary  can  be  ascertained 
by  growing  a  set  of  plants  in  water  cultures  and 
omitting  from  successive  bottles  each  element  in  turn. 
Taking,  for  example,  in  bottle  i,  the  standard 
solution  which  we  have  already  employed ;  in  2  the 
potassium  nitrate  may  be  omitted,  thus  giving  a 
solution  without  nitrogen  (the  potassium  will  be  still 
furnished  by  the  potassium  phosphate) ;  in  3  the 
potassium  phosphate  is  omitted,  to  get  a  solution  con- 
taining no  phosphoric  acid ;  in  4  the  potassium 
phosphate  and  nitrate  are  replaced  by  the  corresponding 
sodium  salts  ;  and  so  on.  The  seedling  barley  or  maize 
plants  are  inserted  in  the  usual  way,  with  the   result 


III.]  ELEMENTS  NECESSAR  V  TO  PLANT  GRO  WTH  5 1 

shown  in  the  illustration,  from  which  it  will  be  seen  that 
only  soda  and  perhaps  chlorine  can  be  omitted  without 
some  injury  to  the  plant.  When  either  nitrogen, 
phosphoric  acid,  or  potash  is  omitted,  growth  ceases  as 
soon  as  the  food  supply  in  the  seed  is  exhausted ;  in  the 
absence  of  sulphuric  acid,  or  magnesia,  or  lime,  growth 
continues,  though  it  becomes  rather  abnormal ;  but  soda, 
silica,  and  sometimes  chlorine  can  be  left  out  without 
causing  any  difference  to  the  growth.  If  no  iron  is 
added  to  the  solution,  the  leaves  of  the  seedling  plant 
soon  become  very  pale  in  colour;  the  new  shoots  in 
particular  will  be  of  a  pale  straw  colour  and  possess  no 
substance,  until  in  a  short  time  the  whole  plant  perishes. 
But  if  a  few  drops  of  ferric  chloride  are  added  to  the 
solution,  on  the  following  day  the  veins  of  the  leaves 
will  be  seen  to  be  turning  green  again,  and  the  whole  of 
the  leaf  will  rapidly  follow  suit.  In  some  way  an  iron 
salt  is  necessary  to  the  formation  of  the  chlorophyll — the 
green  colouring  matter  of  the  leaf — without  which  no 
assimilation  can  take  place.  We  may  extend  the 
method  of  water  cultures  to  ascertain  in  what  state  of 
combination  the  various  elementary  constituents  must 
be  presented  to  the  plant  if  they  are  to  be  utilised  by 
the  plant.  In  our  experiment,  for  example,  we  have 
used  a  nitrate  as  the  source  of  nitrogen,  and  seen  that 
the  plant  can  take  in  this  compound  of  nitrogen  and 
elaborate  from  it  all  the  proteins  and  other  complex 
nitrogenous  bodies  it  contains ;  which  of  the  numerous 
other  compounds  of  nitrogen  can  be  similarly  utilised  ? 
As  a  matter  of  fact,  only  a  very  few,  and  those  of  the 
simplest  type  of  construction,  can  be  so  utilised ;  in 
addition  to  the  nitrates,  plants  will  take  in  the  salts  of 
ammonia  and  a  very  few  bodies  like  asparagin — i.e.,  simple 
amino  bodies  such  as  they  build  up  themselves  at  a  very 
early  stage  from  the  still  simpler  nitrates  and  ammonia 


52  THE  WORK  OF  THE  ROOTS  [chap. 

salts  taken  from  the  soil.  Phosphorus,  again,  must 
enter  the  plant  as  a  phosphate  or  phosphoric  acid ; 
sulphur  as  a  sulphate,  for  sulphites,  sulphides,  and  many- 
other  compounds  of  sulphur  are  not  only  useless  but 
poisonous.  The  bases  found  in  the  plant  enter  as  the 
neutral  salts  of  sodium,  potassium,  calcium,  magnesium, 
etc. ;  in  fact,  for  all  the  elements  except  chlorine  we  may 
say  that  the  plant  prefers  or  even  requires  the  ordinary 
most  highly  oxidised  compound  of  each  element.  It 
will  thus  be  seen  that  the  plant  behaves  towards  its 
nitrogen  and  its  ash  constituents  just  as  it  does  towards  its 
carbon  compounds — i.e.^  it  begins  with  a  simple  oxidised 
compound,  reduces  it  by  splitting  off  oxygen,  and  builds 
it  up  into  a  variety  of  bodies  of  great  complexity 
possessing  more  potential  energy  than  the  initial  com- 
pounds out  of  which  they  have  been  wrought.  It  is 
easy  to  understand  that  a  plant  must  take  up  the 
mineral  substances  which  are  found  in  its  ash  from  the 
soil,  because  it  is  in  contact  with  no  other  source  of  such 
a  constituent  as  phosphoric  acid.  The  case  is,  however, 
different  as  regards  nitrogen,  because  the  plant  when 
growing  is  surrounded  by  the  atmosphere,  four-fifths  of 
which  consists  of  nitrogen  in  a  free,  uncombined  state.  It 
has  therefore  been  a  question  of  much  interest — one  that 
has  always  been  in  dispute,  and  is  still  not  regarded  as 
settled  by  some  people — whether  the  plant  is  not  able  to 
utilise  some  of  this  vast  stock  of  free  nitrogen  and  draw 
a  part,  if  not  the  whole,  of  the  combined  nitrogen  it 
contains  from  such  a  source.  The  evidence  is,  however, 
strongly  against  the  view  that  plants  are  in  any  way 
able  to  "  fix  "  or  bring  into  combination  the  nitrogen  gas 
of  the  air,  except  in  one  or  two  special  cases  where  the 
process  is  really  effected  by  certain  bacteria  living  in 
partnership  with  the  plants ;  the  general  run  of  plants 
are    supposed    to  be  wholly  dependent   on   combined 


III.]     PLANTS  REQUIRE  COMBINED  NITROGEN       53 

nitrogen  which  they  derive  from  the  soil.  In  the  water- 
culture  experiments,  for  example,  we  see  how  incapable 
of  growing  the  plant  becomes  if  the  nitrate  is  omitted 
from  the  culture  liquid  ;  and  the  same  thing  occurs  if  the 
conditions  are  made  more  normal  by  growing  the  plants 
in  an  artificial  nitrogen-free  soil.  Most  careful  experi- 
ments have  also  been  made,  in  which  plants  are  grown  in 
prepared  soil  and  the  nitrogen  present  in  the  seed  and 
soil  at  the  beginning  is  compared  with  the  nitrogen 
contained  at  the  end  in  the  plant  and  soil,  with  the 
result  that  no  gain  from  the  atmosphere  is  detectable. 
These  experiments,  however,  are  very  difficult,  and 
subject  to  a  large  experimental  error ;  moreover,  it  has 
been  argued  that  the  artificial  conditions  result  in  such 
a  diminished  vigour  that  the  plant  has  no  longer  the 
vitality  necessary  to  fix  the  atmospheric  nitrogen. 

Such  arguments  are,  however,  disposed  of  by  another 
type  of  experiment,  in  which  a  series  of  plants  growing 
in  sand  supplied  with  the  necessary  mineral  constituents 
are  given  varying  amounts  of  nitrate,  with  the  result 
that,  up  to  a  certain  limit,  the  growth  is  strictly  propor- 
tional to  the  amount  of  nitrogen  supplied.  The  same 
kind  of  trial  has  been  made  in  the  field  ;  for  example,  at 
Rothamsted  one  plot  of  mangolds  was  given  a  small, 
amount  of  ammonium  salts  to  supply  enough  nitrogen 
to  start  the  plant  into  full  vigour;    Table  IV.,  however, 

Table  IV.— Produce  of  Mangold  Roots,  Rothamsted. 
27  Years'  Average. 


Manure. 

Tons  per  acre. 

Return  for  1  lb. 

Nitrogen. 
Tons  per  acre. 

Minerals,  no  Nitrogen 
„         7-8  lb.     ,.     .        . 
94  lb.    „     . 

4-55 
14-60 

0-l8 
o«ii 

54  THE  WORK  OF  THE  ROOTS  [chap. 

shows  that  the  extra  growth  thus  produced  above  that 
on  the  plot  wholly  without  nitrogen  was  small,  and  is 
closely  proportional  to  the  nitrogen  supply,  when  com- 
pared with  the  increase  of  crop  produced  by  a  much 
larger  amount  of  nitrogen. 

Indeed,  the  Rothamsted  experimental  results,  taken 
collectively,  are  only  consistent  with  the  supposition  that 
the  ordinary  farm  crop  obtains  the  whole  of  the  nitrogen 
it  requires  from  the  soil ;  on  all  the  plots  receiving  no 
nitrogen  whatever,  the  crop  (leguminous  plants  alone 
excepted)  has  been  reduced  to  a  very  low  level ;  indeed, 
it  is  only  due  to  the  large  stock  of  nitrogen  originally 
present  in  the  soil  and  to  certain  recuperative  actions 
at  work  there  that  it  is  maintained  at  all.  The  difficulty 
about  this  conclusion — that  plants  utilise  only  the  com- 
bined nitrogen  in  the  soil — is  to  understand  how  many  of 
the  rich  virgin  soils,  black  with  organic  matter  down  to 
a  depth  of  nine  or  ten  feet,  can  have  accumulated  the 
nitrogen  they  contain.  If  the  plant  contains  only  the 
nitrogen  it  has  taken  from  the  ground,  there  can  be  no 
gain  of  nitrogen  when  the  plant  falls  back  to  the  ground, 
however  numerous  the  generations  in  which  such  a 
vegetative  cycle  may  be  repeated.  Some  other  agencies 
than  the  mere  growth  and  decay  of  plants  must  have 
been  at  work  fixing  nitrogen,  and  these  will  be  discussed 
later. 

While  we  are  dealing  with  the  nutrition  of  the  plant 
by  means  of  the  root,  one  other  point  requires  considera- 
tion :  the  analysis  of  the  ash  of  any  given  plant  shows 
a  very  similar  composition,  wherever  or  on  whatever 
kind  of  soil  the  plant  has  been  grown.  For  example,  the 
ash  of  wheat  straw  (see  Table  V.)  is  characterised  by  a 
high  percentage  of  silica  and  a  low  one  of  lime ;  this 
will  be  the  case  whether  the  wheat  is  grown  on  a  sandy 
or  a  chalky  soil,  and  is  in  sharp  contrast  to  the  ash  of 


III.] 


COMPOSITION  PER  CENT.  OF  ASH 


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56  THE  WORK  OF  THE  ROOTS  [chap. 

clover,  which  contains  no  silica  but  much  lime,  or  the 
ash  of  mangolds,  which,  while  containing  a  good  deal  of 
lime,  is  specially  rich  in  potash  and  soda.  These  specific 
differences  would  seem  to  show  that  the  roots  of  a  plant 
possess  a  power  of  selection,  so  that  they  can  in  the  one 
case  take  in  silica  and  in  the  other  reject  it ;  or,  again, 
that  they  can  take  in  potash  in  preference  to  soda,  or 
both  rather  than  the  lime  which  is  so  much  more 
abundant  in  most  soils.  It  is  a  mistake,  however,  to 
attribute  this  selective  power  to  the  roots ;  it  really  resides 
in  the  active  growing  cells  of  the  plant ;  the  root  hairs 
(and  they  are  the  active  absorptive  organs  of  the  plant, 
both  for  water  and  the  nutrients  coming  in  with  the 
water)  allow  the  passage  of  whatever  dissolved  substances 
are  presented  to  them  until  the  sap  within  possesses 
the  same  concentration  as  the  water  outside  the  plant. 
When  this  stage  has  been  reached,  no  more  can  enter, 
until  the  living  cells  of  the  plant  by  withdrawing  some 
of  the  material  for  constructive  purposes  lower  the 
concentration  of  the  sap  in  that  particular  substance. 
Potash,  for  example,  accumulates  in  a  plant  rather  than 
soda,  even  though  there  may  be  more  soda  in  the  soil, 
because  the  plant's  cells  keep  utilising  the  potash  and 
taking  it  out  of  the  sap  solution;  whereas  a  small 
quantity  of  soda  maintains  the  plant  sap  as  saturated 
as  the  soil  water  outside,  because  so  little  of  it  is 
required  by  the  plant's  cells.  But  though  the  active 
agency  resides  in  the  growing  cells,  and  not  in  the 
roots,  the  result  is  the  same — the  plant  as  a  whole 
does  exert  a  selective  action  on  the  elements  of  nutrition 
presented  to  it,  so  that  all  plants  of  the  same  kind  have 
a  certain  characteristic  ash  composition,  in  which 
differences  of  soil,  season,  manuring,  etc.,  do  not 
cause  wide  variations.  Furthermore,  plants  of  different 
species  possess  sharply  differing  characteristics  in  the 


III.]  COMPOSITION  OF  CROPS  57 

composition  of  their  ash.  These  differences  never 
extend  to  the  entire  absence  from  one  plant  of  an 
element  present  in  another  :  all  plants  contain  the  same 
elements,  except  silica,  which  is  abundant  in  some  plants 
and  entirely  absent  from  others,  though,  as  far  as  is 
known,  it  is  unessential  to  either. 

The  common  idea  that  certain  plants  flourish  only 
in  particular  soils  because  they  find  there  some  par- 
ticular constituent  which  is  elsewhere  lacking,  is 
therefore  erroneous ;  we  might  almost  say  that  all  soils 
contain  all  the  constituents  that  plants  need,  so  that  as 
far  as  food  goes,  where  one  plant  can  grow  any  other  should 
be  equally  possible.  It  is  true  that  the  excess  or 
deficiency  of  certain  soils  in  some  particular  constituent 
renders  them  more  or  less  appropriate  to  one  crop  or 
another,  but  the  circumstances  which  determine  the 
association  of  a  given  plant  with  a  particular  soil  are 
more  often  questions  of  texture,  temperature,  and  water 
supply,  than  of  absolute  nutrition,  as  represented  by  the 
greater  or  less  provision  in  a  soil  of  a  particular  con- 
stituent of  the  plant. 

Although  the  composition  of  different  plants  does 
not  vary  within  very  wide  limits,  and  though  as  we  shall 
see  later  the  composition  does  not  throw  much  light  on 
the  suitability  of  particular  soils  for  certain  crops,  it  is 
yet  desirable  to  know  what  quantities  of  plant  food  are 
removed  from  the  soil  by  the  ordinary  farm  crops. 

Only  average  figures  can  be  given  because  of  the 
variations  set  up  by  season  and  soil,  but  in  Table  VI. 
are  set  out  the  amounts  of  the  chief  nutrient  constituents 
contained  in  the  usual  farm  crops,  based  chiefly  upon 
analyses  at  Rothamsted  : — 


[Table  VI. 


S8 


THE  WORK  OF  THE  ROOTS       [chap.  hi. 


Table  VI.— Composition 

OF  Average  Crops  per 

Acre. 

Dry 

matter. 

Nitrogen. 

Phosphoric 
Acid. 

Potash. 

Wheat  Grain,  30  bushels   . 
Wheat  Straw,  i|  tons 

Total        . 

Barley  Grain,  40  bushels  . 
Barley  Straw,  i  ton  . 

Total 

Oat  Grain,  45  bushels 
Oat  Straw,  2^  tons    . 

Total 

Maize  Com,  50  bushels     . 
Maize  Forage,  3  tons 

Total 

Meadow  Hay,  i^  tons 

Red  Clover  Hay,  2  tons    . 

Swedes,  Roots,  20  tons      . 

Mangolds,  Roots,  30  tons . 

Potatoes,  8  tons 

1530 
2650 

34 
16 

14 
7 

9 
20 

4180 

50 

21 

29 

1750 
2080 

35 
14 

16 

5 

10 
26 

3830 

49 

21 

36 

1630 
2350 

34 
18 

13 
6 

9 

37 

3980 

52 

19 

46 

2500 
3000 

46 
25 

17 
14 

II 
50 

5500 

71 

31 

61 

2820 
3760 
4700 
7900 
4480 

49 

ICO 

100 

131 

61 

12 
25 
23 
48 

28 

51 

83 

90 

290 

100 

CHAPTER  IV 

CHANGES  OF  COMPOSITION  WITHIN   THE  PLANT 

The  Manufacturing,  Resting,  and  Spending  Stages  in  a  Plant's 
Development.  The  Course  of  Nutrition  and  Migration  in 
the  Growth  of  Wheat.  The  Ripening  of  the  Grain.  Storage 
and  Migration  in  Root  Crops.  Removal  of  Food  Materials 
from  the  Leaves  of  Trees  as  they  Ripen.  The  Ripening  of 
Fruit.  Effect  of  Soil  and  Climate  upon  the  Composition  and 
Quality  of  the  Crop. 

A  REFERENCE  has  already  been  made  to  the  fact  that 
the  starch  manufactured  in  the  chlorophyll-containing 
cells  of  the  leaf  is  not  allowed  to  remain  there  long,  but 
is  transformed  into  soluble  sugars  by  the  action  of 
enzymes  or  ferments  also  present  in  the  leaf,  and  is 
then  moved  on  to  some  other  part  of  the  plant,  where 
it  is  stored  as  sugar  or  as  starch  again  until  it  is  wanted. 
It  is  now  necessary  to  consider  such  processes  rather 
more  generally  from  the  point  of  view  of  the  whole 
economy  of  the  plant,  in  which  operations  of  manu- 
facture, transport,  storage,  and  remigration  take  place, 
giving  rise  to  the  stages  we  call  growth  or  resting  or 
ripening.  In  some  plants  the  various  operations  we 
have  just  enumerated  can  be  seen  very  distinctly  taking 
place  in  succession;  this  is  particularly  the  case  with 
bulbous  plants,  which  have  evolved  the  plan  of  laying 

69 


6o  CHANGES  OF  COMPOSITION  [chap. 

up  a  large  reserve  store  of  food  to  carry  the  plant  in  a 
resting  condition  through  periods  of  summer  drought. 
Perhaps  the  most  instructive  example  is  afforded  by  the 
Autumn  Crocus  {Colchicmn)^  which,  as  a  wild  plant,  is 
abundant  in  the  west  country  pastures,  and,  with  its 
allies,  is  also  not  uncommon  in  gardens.  The  noticeable 
feature  about  the  autumn  crocus  is  that  it  throws  up 
its  blossoms  without  any  leaves  in  the  autumn,  the 
flowers  being  followed  sometime  later  by  the  seed  pod  ; 
in  March  the  leaves  appear  and  grow  to  a  considerable 
size  without  any  sign  of  blossom.  During  the  growth 
of  the  leaves,  a  corm  (the  equivalent  of  the  bulb 
in  this  case)  is  formed,  and  grows  to  a  considerable 
size,  being  packed  with  starch,  etc.,  which  is  manu- 
factured by  the  leaf  and  transferred  for  storage  to 
the  corm.  Towards  the  end  of  June  the  leaves  die 
down  entirely  and  the  plant  goes  to  rest,  still,  how- 
ever, respiring  slowly  in  the  corm,  such  respiration  being 
a  necessary  condition  of  vitality.  With  the  autumn, 
however,  the  corm  renews  its  vigour,  the  flower  stems 
are  thrown  up,  but  all  the  material  therein,  as  well  as 
that  required  to  support  the  rapid  respiration  which 
attends  the  flowering  period,  is  drawn  from  the  material 
that  had  previously  been  stored  in  the  corm.  As  far  as 
the  plant  goes,  this  is  a  period  of  pure  spending  of  its 
accumulated  resources,  because  the  plant  no  longer 
possesses  any  leaves  or  other  green  organs  capable  of 
assimilation;  the  corm  itself  becomes  depleted,  but 
under  favourable  conditions  a  quantity  of  seed  is  formed, 
capable,  by  its  food  store,  of  reproducing  the  species 
and  giving  it  a  start  in  the  next  generation.  The  corm 
itself  also  retains  enough  food  material  to  carry  it 
through  the  winter  and  start  new  leaves  which  will 
build  up  the  reserve  afresh  in  the  spring,  thus  repro- 
ducing the  same  plant   in  the  following  year.     With 


IV.]  DEVELOPMENT  OF  WHEAT  6i 

other  bulbous  plants  the  sequence  of  getting  and  spend- 
ing is  not  perhaps  so  clearly  seen  as  in   the  autumn 
crocus :  in  the  common  crocus,  for  example,  the  leaves 
are  almost  contemporaneous  with  the  flowers  ;  with  the 
tulip    and    daffodil    it    requires   closer  observation   in 
order  to  realise  that  the  processes  of  manufacture  and 
storage  are  quite  distinct  from  those  of  flower  and  seed 
formation,  because  the  two  are  going  on  almost  at  the 
same  time.     Just  in  the  same  way,  even  with  annual 
plants  like  wheat,  we  may  distinguish  the  processes  and 
periods  of  manufacture  and   storage,  followed  by  the 
later    process    of    migration,    when    the    accumulated 
material  is  stored  up  afresh  in  the   seed  as  a  reserve 
wherewith  to  give  the  young  plant  a  start  in  life.     The 
course  of  existence  of  a  wheat  plant  merits  particular 
examination  from  this  point  of  view,  and  the  changes 
taking  place  may  be  studied  in  some  detail  because 
they  are  typical  of  much  of  what  is  going  on  in  all 
plants.     The  starting-point  is  the  seed  sown  in  autumn ; 
after  it  germinates  the  blades  grow  2  or  3  inches  high, 
but   there   they  generally  remain   for  the  rest  of  the 
winter,  often  indeed  appearing  to  dwindle  because  of 
the  way  the  leaves  lie  down  with  the  first  frost.     But 
though  the  growth  above  ground  is  almost  at  a  standstill, 
the  material  that  is  being  formed  by  assimilation  is  used 
up  for  the  formation  of  roots,  which  are  pushing  deeper 
into  the  soil  all  through  the  winter.     It  is  at  this  stage 
that  the  foundation  is  laid  for  the  future  crop,  and  a  wet 
autumn  and  winter,  by  limiting  the  aeration  of  the  soil, 
causes  a  restricted  root  development  which  is  always 
followed  by  a  poor  yield.     For  example,  if  we  compare 
the  crops  upon  three  of  the  manured  plots  at  Rotham- 
sted,  we  get  the  following  results  for   the  ten  wettest 
and    the   ten    driest   winters    (November    to    January 
inclusive)  between  1852  and  1904: — 


62 


CHANGES  OF  COMPOSITION 


[chap. 


Rainfall. 
Four  months, 
Nov.  to  Feb. 

Yield  of  Grain.    Bushels. 

Mean. 

Plot  6. 

Plot  7. 

Plot  8. 

Wet  winters    . 
Dry  winters    . 

13-005 
5-786 

19-4 
28-0 

27.1 
37-1 

32 -o 
38-9 

26.2 
34-7 

Thus  there  was  on  the  average  a  better  crop  by  30  per 
cent,  after  the  dry  than  after  the  wet  winters,  and  this 
may  largely  be  set  down  to  the  restricted  root  develop- 
ment in  the  wet  seasons.  Of  course,  a  wet  autumn  in 
other  ways  acts  very  prejudicially  to  the  future  crop  of 
wheat,  partly  by  interfering  with  the  sowing  at  the 
proper  time  and  partly  by  washing  much  of  the  soluble 
plant  food  in  the  soil  down  below  the  reach  of  the 
plant. 

When  the  winter  is  past  and  the  wheat  begins  to 
grow  again,  it  enters  upon  its  most  active  period  of 
drawing  nutriment  from  the  soil  and  of  assimilating 
carbon  from  the  atmosphere ;  the  products,  as  fast  as 
they  are  manufactured  in  the  leaf,  are  moved  off  and 
stored  up  in  the  stems  of  the  plants.  As  the  lower 
leaves  age  and  begin  to  yellow  off  and  die,  the  various 
valuable  materials  they  contain,  not  only  the  carbo- 
hydrates that  have  been  formed  by  assimilation,  but  the 
nitrogen,  phosphoric  acid,  and  potash  which  are  b^ing 
employed  in  the  vital  processes,  are  very  largely  with- 
drawn and  passed  on  to  some  more  active  part  of  the 
plant.  These  substances  are  of  too  great  importance  to 
the  plant  to  be  wasted  by  being  allowed  to  remain  idle 
in  dead  tissue  or  to  be  lost  to  the  plant  when  the  leaf 
falls,  consequently  they  are  taken  back  into  the  active 
parts  of  the  plant  as  far  as  possible  before  the  leaf  dies. 
Throughout  the  months  of  March  to  June,  the  processes 
of  manufacture  and  storage  alone  are  going  forward  ;  but 


IV.]  THE  FILLING  OF  THE  GRAIN  63 

as  the  wheat  comes  into  flower,  these  processes  begin  to 
slow  down :  in  particular,  the  feeding  of  the  plant  upon 
the  soil  becomes  less.  This  may  be  traced  in  the 
diagram.  Fig.  15,  which  shows  the  amounts  of  dry 
matter,  nitrogen,  and  phosphoric  acid  contained  in  the 
whole  plant  and  in  the  grain  separately,  from  the  time 
when  the  grain  could  first  be  separated — about  a 
fortnight  after  flowering — up  to  the  time  of  harvest.  The 
curves  show  that  at  the  starting-point  the  plant  had 
reached  about  90  per  cent,  of  its  ultimate  dry  weight, 
but  that  it  had  acquired  only  about  75  per  cent,  of  the 
nitrogen  and  less  than  70  per  cent,  of  the  phosphoric 
acid  that  were  finally  present  in  the  plant.  It  should, 
however,  be  noted  that  these  figures  take  no  account  of 
the  root,  which  doubtless  itself  contained  some  of  the 
nitrogen  and  phosphoric  acid  that  later  found  their  way 
into  the  above-ground  parts  of  the  plant.  The  earlier 
experiments  of  Pierre,  conducted  in  the  hotter  and  drier 
climate  of  France,  would  indicate  that  the  wheat  plant 
ceases  entirely  to  take  nutriment  from  the  ground  at  a 
much  earlier  date — soon  after  flowering ;  whereas  in  the 
experiments  quoted  the  processes  of  nutrition  never 
stopped  until  the  whole  plant  was  ripening  off  for 
harvest.  Assimilation  is  also  shown  to  be  going  on  as 
long  as  the  plant  possesses  any  green  leaf  tissue ;  during 
the  last  fortnight  or  so  before  cutting,  the  dry  weight 
did  not  increase  at  all ;  in  fact,  it  decreased  because  the 
burning-up  of  carbohydrate  by  respiration  continued  to 
go  on  and  towards  the  end  caused  losses  which  out- 
weighed the  diminishing  gains  by  assimilation.  From  the 
time  of  the  formation  of  the  grain,  the  most  important 
process  going  forward  was  the  migration  into  the  seed  of 
the  material  that  has  previously  been  stored  in  the 
stem.  It  will  be  seen  that  the  gain  of  dry  matter,  and 
particularly  of  nitrogen  and   phosphoric   acid,  by   the 


64  CHANGES  OF  COMPOSITION  [chap. 

grain  during  this  period  was  far  greater  than  would  be 
accounted  for  by  what  the  plant  had  taken  up  meantime 
from  either  the  soil  or  the  air.  In  fact,  at  the  finish, 
although  only  two-fifths  of  the  whole  dry  matter  of  the 
plant  was  collected  in  the  grain,  yet  80  per  cent,  of  the 
nitrogen  and  70  per  cent,  of  the  phosphoric  acid  of  the 
whole  plant  had  been  transferred  to  it.  This,  of  course, 
is  only  another  example  of  the  process  which  we  have 
already  noted  as  taking  place  when  the  leaves  died  off 
one  by  one — the  valuable  materials  they  contained  were 
constantly  transferred  to  the  still  living  and  active  parts 
of  the  plant.  In  the  same  way,  the  seed  is  just  that  part 
of  the  plant  which  has  to  carry  on  its  life  in  the  next 
generation,  and  it  is  endowed  with  as  much  as  possible 
of  the  material  that  has  been  with  difficulty  accumulated 
by  the  parent  plant. 

During  the  formation  of  the  grain  itself,  three  stages 
may  be  distinguished ;  for  about  three  weeks  from  the 
time  of  flowering  the  main  process  that  is  going  forward 
is  the  formation  of  what  later  become  the  outer 
envelopes  of  the  grain,  in  a  thick  and  fleshy  form  which 
they  lose  afterwards.  This  "  pericarp,"  as  it  is  called,  is 
richer  than  the  ultimate  grain  in  nitrogen  and  ash, 
though  the  ash  does  not  contain  so  much  phosphoric 
acid.  There  is,  of  course,  no  sudden  break  which  marks 
the  oncoming  of  the  second  stage,  but  towards  the  end 
of  the  third  week  the  formation  of  the  pericarp  has 
ceased  and  the  filling  in  of  the  endosperm  begins  to  be 
the  main  process  going  forward ;  as  this  progresses,  the 
pericarp  is  denuded  of  the  material  already  accumulated 
there  and  becomes  reduced  to  the  series  of  tough,  dry 
membranes,  with  which  we  are  familiar  later  as  the  chaff 
and  skin  of  the  grain.  The  filling  of  the  endosperm 
continues  for  nearly  three  weeks,  and  the  most  notable 
feature  is  that  the  material  migrated  by  the  plant  from 


•bry 

I  Matter. 

Grammes. 


MU 

^ 

«4/vr 

p^ 

TTf« 

WJVH. 

(it 

^ 

^ 

y 

"^ 

/ 

V.//V  > 

ihOLL 

;5^ 

^ 

^^ 

/ 

yo 
80 
70 

60 
SO 

/ 

"^ 

^ 

t*.-^ 

t*- 

^ 

^ 

NT  — 

P.05_ 

..*.".' 

h?-'- 

....- 







'••"■" 

,^ 

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«22 

MN 

40 

30 
20 

^ 

X^ 

^ 

y-i. 

C^ 

^ 

y 

.-i> 

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^ 

10 

y 

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^ 

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""^ 

Kitrogen 
and  P2O5. 

Grammes. 
1-3 


0.9 
0.8 

•7 


3  6  9  12         IS         18         21        24         27        30         33  36  days. 

Fig.  15.— Diagram  showing  the  Migration  of  Plant  Constituents 
INTO  THE  Grain  of  Wheat  during  the  last  Five  Weeks  before 
Harvest. 


{Face  page  64. 


IV.]  QUALITY  IN  WHEAT  6$ 

day   to  day   possesses  an  almost  constant  composition 
throughout  the  process,  containing  the  same  proportion 
of  carbohydrates,  nitrogen  compounds,  phosphoric  acid, 
etc     It  is  a  mistake  to  suppose  that  the  proteins  which 
form  the  gluten  are  filled  in  first,  and  that  starch  only 
enters  towards  the  later  stages  of  filling  and  ripening. 
But  though  any  particular  lot  of  wheat  does  in  this  way 
pass  on  a  constant  mixture  of  materials  from  its  reserves 
to  the  grain,  there  will  be  certain  differences  in  the  type 
when  different  varieties  or  the  same  variety  grown  in 
different  climates   or  soils  are   compared.     Each  plant 
in  a  given  field  of  wheat  possesses,  as  it  were,  a  mould 
wherewith  to  fashion   the  material  it  passes  on  to  its 
grain,  and  the  form  of  the  mould  is  determined  by  the 
variety  and  the  environment — soil,  climate,  season,  etc. 
Wheats  grown  in  certain  climates,  such  as  those  prevail- 
ing  in    Kansas,  Manitoba,  or    Hungary,   are   specially 
nitrogenous  (glutenous  or  "  strong  "  wheats),  but  this  is 
not   due   to   the   hot    dry   climate   shutting   down   the 
migration   process  prematurely  before  the  later  stages 
are  reached,  when   starch   only   enters   the   grain,   but 
to   the   fact  that   the    environment — climate   and   soil, 
together  with  the  varieties    appropriate  to   such   con- 
ditions— causes    the   wheat    to    make    that    which  we 
have   called    its    mould    on   a    nitrogenous   type.      It 
should,    however,    be    noted     that     though    variations 
due  to  environment  do  occur,  the  composition  of  the 
grain  of  wheat  is  singularly  constant.     It  has  already 
been  explained  how  all  plants  react  against  variations 
in  the  composition  of  the  soil  and  select  the  particular 
constituents  appropriate  to  their  nutrition ;  so,  similarly, 
when   the   migration   process   begins,  the   plant   again 
effects  a  re-sorting  of  the  material  accumulated,  and  only 
passes  on  its  usual  constituents  to  the  grain.     Thus  the 
straw  will  always  vary  much  more  in  composition  from 

E 


66  CHANGES  OF  COMPOSITION  [chap. 

soil  to  soil,  climate  to  climate,  etc.,  than  the  grain. 
When  the  filling  of  the  endosperm  has  been  pretty  well 
completed — which  is  about  a  week  or  a  fortnight  before 
the  time  at  which  the  grain  would  usually  be  regarded 
as  fit  to  cut — the  ripening  process  sets  in,  and  this  is,  in 
the  main,  characterised  by  a  drying  up  of  what  has  been 
previously  accumulated.  During  this  last  fortnight  the 
dry  matter  of  the  grain  increases  little,  if  at  all ;  certainly 
it  is  stationary  in  amount,  or  even  declining,  during  the 
last  week,  and  water  is  being  constantly  removed.  The 
main  feature,  then,  of  the  ripening  process  is  desiccation  ; 
at  the  same  time,  other  rearrangements  in  the  chemical 
nature  of  the  constituents  can  be  seen :  non-protein 
nitrogen  compounds  pass  over  into  the  protein  state, 
and  the  proportion  of  sugar  falls,  but  these  changes  are 
not  large  compared  with  the  drying  up  which  is  taking 
place.  It  would  appear  that  it  is  possible  to  cut  wheat 
rather  earlier  than  the  usually  recognised  stage  of 
ripeness  without  any  loss  of  weight  in  the  crop,  by 
which  means  losses  by  shedding,  birds,  etc.,  might  also 
be  reduced,  the  only  practical  question  being  whether 
the  final  drying  process  can  be  better  effected  while  the 
wheat  is  standing,  or  after  it  is  cut  and  shocked  or  even 
put  into  stack. 

Certain  other  practical  consequences  depend  upon 
this  migration  process ;  for  example,  if  from  any  cause 
the  crop  is  cut  before  migration  is  complete,  then  the 
straw  will  be  so  much  richer  in  both  fertilising  and  food 
constituents.  Similarly,  in  bad  seasons,  when  ripening 
is  imperfect,  the  migration  process  is  always  found  to  be 
much  less  complete,  so  that  a  higher  proportion  of  the 
accumulated  material  is  left  in  the  straw.  Heavy 
manuring,  again,  especially  manuring  of  a  nitrogenous 
character,  is  always  attended  by  a  less  complete  migra- 
tion, leaving  the  straw  by  so  much  the  richer.     Straw 


IV.] 


EFFECT  OF  SEASON  UPON  WHEAT 


67 


from  the  northern  parts  of  the  country,  where  growth  is 
more  prolonged  and  ripening  less  complete,  is  always 
of  better  feeding  value  than  straw  grown  where  drier 
and  hotter  summers  prevail.  Some  of  these  points 
are  illustrated  in  Table  VII.,  which  shows  composi- 
tion of  grain  and  straw  and  the  ratio  between  them, 
for  the  wheat  grown  on  three  of  the  Rothamsted  plots 
in  two  sharply  contrasted  seasons — 1852,  which  was  cold 
and  wet ;  and  1863,  which  was  hot  and  dry  : — 

Table  VII.— Composition  of  Wheat  Crop  in  Wet  and  Dry 
Seasons. 


Plot. 

3 

Un- 
manured. 

2 

Dung. 

7 

Artificial 
Manures. 

Weight  per  bushel,  lb.  . 

r 
■  I 

1852 
1863 

56-6 
62.7 

58.2 
63-1 

56-0 
62.6 

Grain  to  loo  Straw 

•{ 

1852 
1863 

53-9 
70.4 

49.6 
67-5 

41.9 
59-4 

Nitrogen  per  cent,  in  Dry  Grain 

•{ 

1852 
1863 

2.08 
1.65 

2-02 

1-52 

2.29 

1-53 

Nitrogen  per  cent,  in  Dry  Straw 

/ 

1852 
1863 

0-57 
0-33 

0'46 
0.25 

0-87 
0-36 

Thus  in  the  wet  year,  1852,  there  is  a  smaller  propor- 
tion of  grain,  and  the  straw  is  left  much  richer  in 
nitrogen  than  in  the  dry  year;  the  heavily  manured 
plots  2  and  7  also  show  a  greater  proportion  of  straw, 
which  is  richer  in  nitrogen  than  that  from  the 
un  manured  plot.  If  we  compare  the  wettest  season 
known  at  Rothamsted,  1879,  with  one  of  the  hottest 
and  driest,  1893,  wie  find  the  proportion  of  grain  to  straw 
on  the  unmanured  plot,  which  on  the  average  is  70  per 
cent.,  varied  from  43  in  the  wet  year  1879,  to  no  in 
the  dry  season  of  1893. 

As  to  the  effect  of  heavy  nitrogenous  manuring,  an 
example  may  be  taken  from  some  of  the  Rothamsted 


68 


CHANGES  OF  COMPOSITION 


[chap. 


barley  plots  in  1905,  where  a  comparison  existed 
between  an  unmanured  plot,  a  plot  receiving  a  very- 
large  amount  of  nitrogenous  wool  waste,  and  a  third 
plot  which  had  received  the  same  amount  of  wool  waste 
a  year  earlier,  so  that  it  had  been  partially  exhausted  by 
the  previous  crop  : — 

Table  VIII.— Composition  of  Barley  Crop.    Rothamsted,  1905. 


Weight 

per 
Bushel. 

Grain  to 
100  Straw. 

Oflfal  Corn 
to  100 
Dressed 
Grain. 

Nitrogen 
per  cent, 
in  Grain. 

No  Nitrogen 
Manured  previous  year 
Manured  same  year     . 

58-0 
57-3 
55-1 

I10-4 
96.6 
72.8 

5-9 
12.5 
34-9 

J. 61 

1-79 
2.42 

Here  the  heavy  manuring  has  evidently  given  rise  to  an 
excessive  proportion  of  straw,  while  the  grain  produced 
was  so  light  that  much  of  it  had  to  be  dressed  out  and 
regarded  as  offal. 

The  principles  which  have  been  illustrated  in  the 
migration  of  the  materials  forming  the  grain  of  wheat 
may  also  be  applied  to  another  case — the  changes  that 
go  on  during  the  later  stages  of  growth  and  ripening  of 
meadow  hay.  Although  the  plants  composing  the 
herbage  of  a  meadow  are  ia  the  main  perennial  and 
not  annual  like  wheat,  the  process  of  nutrition  and 
migration  are  essentially  similar;  carbohydrates  are 
manufactured  from  the  carbon  dioxide  in  the  atmo- 
sphere ;  nitrogen,  phosphoric  acid,  etc.,  are  taken  from 
the  ground  and  elaborated,  until  the  flowering  season  is 
reached,  when  the  movement  of  the  previously  formed 
material  into  the  seed  becomes  the  chief  action  going 
on.  This  is  accompanied  by  an  increasing  change  of 
non-protein  nitrogen  compounds  into  proteins,  a  loss  of 
sugars  by  respiration,  and  a  conversion  of  more  soluble 


IV.]  FOOD  STORAGE  B  V  BIENNIALS  69 

carbohydrates  into  fibre — this  latter  a  rearrangement  we 
did  not  consider  in  the  case  of  wheat  straw.  The 
formation  of  fibre  is  particularly  marked  in  the  final 
stages  when  the  seed  of  the  grasses  gets  ripe,  and  as 
fibre  is  an  inferior  food  material  to  starch,  not  only  less 
digestible  in  itself  but  wasting  much  more  labour  in  the 
process  of  digestion,  there  results  a  considerable  loss  of 
food  value  in  the  material.  All  this  points  to  the 
desirability  of  cutting  hay  in  a  young  state ;  the  loss 
of  weight  in  the  crop  will  not  be  great,  because  for  the 
bulk  of  the  grasses  the  later  stages  of  growth  are  not 
attended  by  much  increase  of  dry  matter,  since  assimila- 
tion becomes  balanced  off  by  respiration,  while  there 
will  be  mechanical  losses  through  the  shedding  of  the 
seed  of  many  plants  ;  at  the  same  time  the  much  smaller 
proportion  of  fibre  in  the  early  cut  hay  permits  of  a  far 
greater  utilisation  of  the  constituents  of  the  hay  by  the 
stock  receiving  it  as  food. 

The  process  of  storage  and  migration  of  the 
accumulated  materials  is  perhaps  most  clearly  seen  in 
the  case  of  the  biennial  root  crops  like  turnips  and 
mangolds  (or  fodder  beet);  the  so-called  bulb — an 
enlarged  stem  in  the  case  of  swede  turnips,  a  root  in  the 
mangolds — is  nothing  more  than  a  storage  organ,  and 
the  whole  work  of  the  first  year's  growth  consists  in 
filling  this  with  the  carbohydrate  assimilated  by  the 
leaves  and  with  the  proteins  which  are  also  there 
elaborated.  In  the  mangold  the  material  stored  is  cane 
sugar,  which  constitutes  about  60  per  cent,  of  the  dry 
matter  of  the  bulb ;  the  dry  matter  itself  varies  from 
about  10-5  per  cent,  in  the  white-fleshed  Globe  variety 
to  over  13  per  cent,  in  the  yellow-fleshed  and  Long  Red 
varieties.  In  the  sugar-beet,  a  form  of  mangold  which 
for  more  than  forty  years  has  been  systematically  bred 
and  selected  for  its  sugar  content,  the    percentage   of 


70  CHANGES  OF  COMPOSITION  [chap. 

sugar  in  the  root  has  been  raised  to  eighteen  or  over. 
The  sugar-content  of  mangold  has  not  been  similarly 
raised,  because  in  its  case  the  seed  parents  have  been 
selected  only  on  a  basis  of  size  and  shape  and  never  on 
the  amount  of  sugar  they  contained,  as  determined  by 
analysis.  The  figures  just  given  for  the  percentage  of 
sugar  and  dry  matter  in  the  mangold  are  of  course  only 
averages;  certain  variations  occur  due  to  the  soil, 
manuring,  and  also  season,  but  perhaps  the  largest 
variation  is  that  due  to  size.  The  larger  the  root  the 
more  watery  it  becomes  ;  mangolds  weighing  approxi- 
mately 2  lb.  apiece  contain  about  2  per  cent,  more  dry 
matter  than  mangolds  averaging  7  lb.  apiece,  above 
which  weight  the  falling  off  in  dry  matter  is  not  so 
marked.  The  age  of  the  mangold  has  not  much 
influence,  it  possesses  much  the  same  composition  all 
the  time  it  is  growing ;  just  as  the  wheat  grain  is  filled 
up  with  material  of  approximately  constant  composition 
from  the  beginning  until  the  end  of  grain  formation,  so 
as  the  mangold  bulb  swells  it  is  being  packed  with 
materials  of  a  fixed  type  as  regards  the  proportions  of 
dry  matter,  sugars,  and  nitrogenous  matter,  this  material 
being  manufactured  in  the  leaf  Certain  changes  do 
occur;  in  the  immature  mangold,  as  in  those  grown 
with  an  excessive  amount  of  manure,  there  is  a  slightly 
smaller  proportion  of  dry  matter,  while  a  much  larger 
proportion  of  the  nitrogen  contained  in  the  root  is 
combined  in  the  form  of  nitrates  and  non-protein 
compounds.  The  nitrates  and  non-protein  compounds 
are  far  from  having  disappeared  when  the  mangold  is 
ripe  in  the  late  autumn,  though  they  are  considerably 
reduced  in  amount.  If  the  mangold  is  allowed  to  grow 
a  second  year  for  seed,  a  reverse  series  of  changes  take 
place ;  the  sugar  is  removed  from  the  cells  of  the  root  in 
order  to  form  the  flowering  stem  and  eventually  the 


IV.]        CHANGES  IN  ROOTS  DURING  STORAGE         71 

seed ;  when  the  seed  is  ripe  the  former  root  will  be 
found  almost  devoid  of  sugar  or  any  other  easily  soluble 
carbohydrate  or  nitrogenous  compound,  little  remains 
except  fibre.  During  the  storage  of  the  mangold  in  the 
clamp  under  ordinary  farming  conditions,  the  ripening 
process  proceeds  during  the  closing  months  of  the  year; 
the  roots  are  of  course  respiring,  and  losing  in  conse- 
quence a  little  sugar,  the  nitrates  and  non-protein 
nitrogen  compounds  are  also  being  partially  replaced  by 
proteins,  but  the  loss  of  material  is  small  as  long  as  the 
temperature  remains  low.  As  soon  as  the  warmer 
weather  of  the  spring  sets  in,  respiration  increases,  some 
degradation  of  the  cane  sugar  into  reducing  sugars 
takes  place,  and  with  sprouting  there  will  also  be 
change  from  the  protein  to  non-protein  nitrogenous 
compounds.  When  mangolds  are  kept  as  late  as  May 
or  June,  there  is  a  very  marked  loss  of  dry  matter  by 
respiration,  which  may  amount  to  as  much  as  one-third 
of  the  original  material,  though  the  loss  will  be 
disguised  by  the  drying-ofif  of  water  which  simultane- 
ously takes  place,  so  that  the  stored  mangold  is 
apparently  as  rich  as  ever  in  dry  matter. 

In  swede  turnips  the  percentage  of  dry  matter  is  about 
the  same  as  in  mangolds,  varying  from  about  10  to  13 
or  over,  according  to  the  variety,  size  of  root,  season  and 
soil,  etc. ;  the  sugars  present  are  different  from  the  cane 
sugars  of  the  mangold,  and  there  is  also  a  considerable 
proportion  of  a  class  of  carbohydrates  known  as  pectins, 
bodies  which  form  a  mucilaginous  jelly  when  boiled 
with  water.  The  changes  which  the  swede  turnip 
undergoes  on  storage  and  when  growing  a  second  year 
for  seed  are  strictly  similar  to  those  taking  place  in  the 
mangold,  the  only  difference  being  that  in  the  forma- 
tion of  the  seed  a  good  deal  of  oil  is  built  up  from  the 
carbohydrates  in  the  storage  material.     White  turnips 


72  CHANGES  OF  COMPOSITION  [chap. 

resemble  swedes  in  composition,  but  are  more  watery ; 
while  potatoes  contain  a  much  higher  proportion  of  dry 
matter,  about  25  per  cent,  and  store  their  carbohydrates 
in  the  form  of  starch.  When  a  potato  set  is  planted  in 
spring  the  starch  is  converted  by  a  diastatic  ferment 
into  sugar,  which  is  moved  off  to  the  growing  shoots, 
where  it  is  used  for  constructing  the  new  tissues ;  as 
growth  progresses  the  cells  will  be  found  to  be  progres- 
sively depleted  of  starch  until  little  more  than  an  empty 
husk  is  left.  A  slow  conversion  of  starch  into  sugar  is 
always  going  on  during  the  storage  of  the  potato,  the 
sugar  being  burnt  up  in  the  respiration  process  which 
keep's  the  plant  alive  though  resting.  It  is  well  known 
that  a  potato  tastes  sweet  if  it  has  been  frosted  or 
exposed  for  some  time  to  a  low  temperature ;  this  is 
because  under  such  conditions  the  respiration  process 
is  almost  suspended,  while  the  enzyme  action  forming 
sugar  still  goes  on  until  there  is  a  perceptible  accumula- 
tion of  sugar  in  the  root. 

Among  perennial  plants  the  movements  of  the 
manufactured  materials  go  on  in  exactly  the  same 
way  as  in  annuals  or  biennials ;  these  movements 
are,  however,  not  so  apparent,  because  of  the  great 
permanent  store  of  food  material  which  exists  in  a 
perennial  plant.  However,  it  is  instructive  to  cut  sec- 
tions of  the  branches  of  trees  at  different  times  of  the 
year  and  test  them  with  iodine  to  show  the  presence  of 
starch  :  in  the  autumn  the  wood,  or  rather  that  part  of 
it  which  is  still  alive  and  active,  is  packed  with  starch, 
but  this  starch  is  moved  out  with  the  first  growth  of  the 
leaves.  It  is  due  to  the  material  thus  stored  that 
cuttings  can  be  struck  from  shoots  cut  off  in  the  autumn  ; 
they  then  contain  easily  available  reserve  material  out 
of  which  the  young  roots  and  the  new  leaves  can  be 
iijs^nufactured  until  the  cutting  attains  an  independent 


IV.] 


THE  FALL  OF  THE  LEAF 


73 


existence.  As  a  rule,  cuttings  cannot  be  struck  from  the 
young  wood  in  the  spring,  because  it  then  possesses  no 
reserve  material  to  draw  upon.  The  leaves  of  deciduous 
trees  afford  another  example  of  the  migration  of  food 
materials ;  when  the  year's  growth  is  over  and  the  tree 
is  preparing  to  shed  its  leaves  for  the  winter,  a  layer  of 
comparatively  large  cork  cells  forms  between  the  stem 
of  the  leaf  and  the  branch,  and  it  is  by  the  rupture  of 
this  dividing  layer  that  the  leaf  becomes  detached  from 
the  tree.  But  long  before  the  leaf  is  detached,  as 
it  begins  to  get  old,  the  valuable  food  materials  it 
contains — the  carbohydrates,  the  nitrogen,  phosphoric 
acid,  and  potash — are  withdrawn  from  the  leaf  and  stored 
away  in  the  stem,  their  place  being  taken  by  comparatively 
abundant  and  valueless  materials  like  lime  and  silica. 
In  consequence,  dead  leaves,  from  a  manurial  point  of 
view,  are  much  poorer  than  living  leaves,  as  the  following 
analyses  (Table  IX.)  of  the  young,  mature,  and  dead 

Table  IX. — Materials  contained  in  500  Leaves  of  the  Plane 
Tree,  gathered  at  different  dates. 


June  18. 

July  16. 

Aug.  22. 

Sept.  7. 

Oct.  8. 

Oct.  24. 

Nov.  6. 

Grms. 

Grms. 

Grms. 

Grms. 

Grms. 

Grms. 

Grms. 

Dry  matter 

142-5 

1847 

182.8 

193-8 

196-2 

148.8 

166.I 

Nitrogen  . 

5-9 

5-9 

4-8 

5-0 

3-4 

1.8 

1.4 

Ash  . 

8.7 

14-6 

17-8 

20-I 

21.3 

18.0 

20-3 

Phosphoric  Acid 

1-3 

1-3 

1-2 

1-2 

0-9 

0-5 

0.6 

Potash 

1.9 

2-1 

2-1 

2-2 

1-6 

10 

0-9 

Lime 

2-5 

5-7 

7-7 

90 

9-5 

8.2 

9.2 

Silica 

0-6 

1.9 

2.9 

3-5 

4-0 

3-9 

3-7 

leaves  of  the  plane  will  show.  The  economy  of  this 
process  is  obvious ;  such  substances  as  the  starch  formed 
by  the  leaf,  the  nitrogen,  phosphoric  acid,  and  potash 
taken  from  the  ground,  are  required  by  the  active  cells  in 
order  to  carry  out  the  vital  processes  of  the  plant ;  they 


74 


CHANGES  OF  COMPOSITION 


[chap. 


are  too  valuable  to  be  cast  aside  in  the  dead  leaf,  and 
are  therefore  carefully  removed  into  the  permanent 
storehouse  of  the  stem  and  roots  during  the  winter 
resting  period. 

In  herbaceous  plants,  which  die  down  to  ground  level 
each  winter,  the  roots  form  the  storage  organs ;  the  stem 
and  leaves  are  depleted  of  their  valuable  constituents 
before  death  sets  in.     For  example.  Table  X.  shows  the 

Table  X. —Material  contained  in  Upper  Growth  of  Hop- 
Plant  AT  TIME  OF  Picking  and  after  Dying  Down. 
Lb.  per  Acre. 


In  Hops 
Sold. 

In  Green  Leaf 

and  Bine  at 

Picking  Time. 

In  Dead  Leaf 
and  Bine  in 
November. 

Returned  to 
Root. 

Nitrogen 

Phosphoric  Acid    . 
Potash  . 
Lime     . 

Lb. 

51 
16 

39 
16 

Lb. 

38 
10 
31 
92 

Lb. 

18 

5 

4 

88 

T  K      Per 
^^-    cent. 
20=  53 

5  =  50 

27  =  87 
4=    4 

material  contained  in  the  parts  of  a  hop  plant  above 
ground  (i)  at  the  time  of  its  maximum  growth  when  the 
hops — i.e.  the  seed  vessels — were  fully  formed  ;  (2)  after 
the  hops  had  been  picked  and  bine  and  leaves  were  dead 
down  to  ground  level,  and  from  the  table  it  will  be  seen 
that  about  one-half  of  the  nitrogen,  one-half  of  the 
phosphoric  acid,  and  nearly  all  of  the  potash  contained 
in  stem  and  leaf  are  removed  and  sent  down  to  the  root 
for  storage  before  the  winter  resting  period  sets  in. 

Similar  cases  of  economy,  storage,  migration,  and 
retranslation  may  be  traced  everywhere  in  the  life- 
histories  of  plants,  the  guiding  fact  being  the  endeavour 
of  the  plant  to  ensure  its  continued  life  and  reproduction 
by  providing  a  sufficient  food  supply  to  start  either  the 
next  generation  (the  seed)  or  the  next  phase  (in  the 
case  of  plants  reproducing  asexually  by  bulbs,  suckers, 


IV.]  RIPENING  01  FRUITS  75 

etc.),  until  it  has  developed  adequate  roots  and  leaves  of 
its  own.  The  broad  chemical  changes  going  on  in  the 
plant,  such  as  the  migration  of  the  starch  from  one  part 
to  another,  preceded  by  its  conversion  into  sugar,  and 
even  such  changes  as  the  desiccation  and  the  conversion 
of  ^-proteins  into  proteins  which  marks  the  ripening  of 
cereals,  are  recognisable  enough;  there  are,  however,  a 
number  of  more  subtle  changes  which  we  are  unable  in 
the  present  state  of  knowledge  to  follow,  though  they  may 
often  be  appreciable  in  the  bulk  as  constituting  what 
the  practical  man  calls  quality.  As  an  example  we 
may  instance  the  ripening  of  fruits  such  as  apples  and 
pears,  which  after  the  migration  and  filling  of  the  fruit 
has  been  completed,  will  to  a  large  extent  go  on 
though  the  fruit  has  been  detached  from  the  tree. 
We  know  the  broad  outline  of  the  process :  the  fruit 
becomes  soft  and  sweet,  often  an  intense  harshness  of 
taste  entirely  disappears,  in  many  cases  fragrant  essences 
and  aromas  develop.  Chemically  we  can  recognise 
that  starch  in  the  unripe  fruit  disappears  because  it  is 
converted  in  sugars,  that  some  of  the  tough  pectin 
bodies  are  also  converted  into  sugars,  as  also  are  the 
very  bitter  tannins,  while  the  agreeable  flavoi-rs  appear 
to  be  due  to  ethers  produced  by  the  union  of  some  of 
the  vegetable  acids  with  alcohol  derived  from  the  partial 
breakdown  of  sugar.  In  fact,  if  the  respiration  of  fruits 
is  deranged  by  putting  them  in  a  jar  containing  no 
oxygen,  the  sugars  will  not  be  converted  into  carbon 
dioxide  and  water,  but  will  be  broken  down  part  of  the 
way  into  the  less  oxidised  form  of  alcohol,  so  that  the 
fruit  may  become  alcoholic  without  any  fermentation  in 
the  ordinary  sense  of  the  word  taking  place.  Fer- 
mentation by  yeast  may  be  regarded  as  a  similar 
case  of  respiration  in  the  absence  of  free  oxygen  ;  the 
yeast  cells  when  placed   in  a  liquid  containing  sugar 


76  CHANGES  OF  COMPOSITION  [chap. 

grow  and  multiply,  and  in  doing  so  they  split  up  by  the 
aid  of  an  enzyme  the  sugar  into  alcohol  and  carbon 
dioxide.  The  yeast  cells  do  not  feed  upon  the  sugar, 
but  they  derive  energy  from  its  splitting  up,  just  as  the 
ordinary  plant  cell  derives  energy  from  the  oxidation  of 
the  sugar  to  carbon  dioxide  and  water  which  takes  place 
during  respiration  in  the  presence  of  oxygen. 

In  discussing  the  changes  which  take  place  in  the 
composition  of  parts  of  the  plant,  we  have  already  had 
occasion  to  point  out  that  the  changes  due  to  variations 
of  environment  are  not  very  great;  the  variations 
induced  by  season  are  much  greater  than  those  due  to 
soil  or  manuring,  as  may  be  illustrated  by  the  figures 
for  the  nitrogen-content  of  the  grain  of  wheat  in 
Table  VII.  In  the  wet  season  of  1852  there  was  about 
2  per  cent,  of  nitrogen  in  the  wheat  grain,  whereas  in  the 
dry  year  of  1863  there  was  little  more  than  1-5  per 
cent. ;  this  season  alone  has  caused  a  difference  of  0-5 
per  cent,  or  25  per  cent,  of  the  total,  whereas  the  greatest 
difference  in  nitrogen-content  between  the  grain  grown 
on  the  wholly  unmanured  plot  and  on  that  which 
receives  an  annual  heavy  dressing  of  farmyard  manure 
only  amounted  to  0-2  per  cent.  As  a  matter  of  fact,  the 
difference  in  the  composition  of  the  crop  brought  about 
by  different  soils  or  different  manuring  may  be  regarded 
as  due  to  the  change  of  climate  which  the  soil  and  manur- 
ing sets  up  for  the  particular  plant.  For  example,  a  heavy 
soil  will  maintain  not  only  moisture  about  the  root  but 
a  moister  atmosphere  about  the  leaves,  and  so  become 
the  equivalent  of  a  wetter  season  ;  on  the  other  hand,  a 
nitrogenous  manure  will  prolong  the  growth  of  a  plant 
and  so  cause  the  ripening  process  to  take  place  under 
other  climatic  conditions  than  those  prevailing  for  an 
earlier  ripening  crop.  If,  however,  the  differences  of 
composition   induced  by  environment — soil,  manuring, 


IV.]  QUALITY  IN  PRODUCE  77 

season,  etc. — are  comparatively  small  as  measured  by 
such  rough  analytical  factors  as  the  percentage  of 
nitrogen,  etc.,  they  are  often  of  extreme  importance 
commercially,  because  they  may  and  generally  do  affect 
those  more  subtle  items  in  the  composition  of  the 
product  summed  up  as  quality.  We  are  as  yet  rarely 
in  a  position  to  define  from  a  chemical  standpoint  what 
constitutes  quality  in  each  particular  case,  still  less  to 
state  the  external  factors  which  determine  it ;  we  may, 
however,  be  sure  that  it  is  not  due  to  the  mere  presence 
or  absence  of  some  particular  ingredient  in  the  soil. 
We  have  seen  that  all  plants  are  fundamentally  made 
up  of  exactly  the  same  things  in  very  similar  propor- 
tions, the  differences  arise  in  the  style  of  the  architecture 
that  the  plant  adopts  in  each  case;  and  if  wheat  is 
"  stronger  "  when  grown  in  one  place  than  another,  it  is 
not  because  in  the  soil  of  the  first  place  there  exists 
some  element  which  is  absent  in  the  soil  of  the  second 
place,  but  because  the  habits  of  the  plant  are  affected 
variously  by  differences  in  the  supply  of  water,  air,  or 
warmth  in  the  two  places.  We  have,  in  fact,  many 
reasons  for  regarding  the  supply  of  water  and  the 
prevailing  temperature  as  the  main  factors  in  determin- 
ing both  the  yield  and  the  quality  of  all  crops,  and  it  is 
from  this  point  of  view  that  it  is  most  profitable  to 
regard  the  soil. 


CHAPTER  V 

THE  ORIGIN   AND   NATURE  OF  SOILS 

The  Weathering  of  Rocks  to  Soil.  Solution  of  Rock  Materials  in 
Water  containing  Carbon  Dioxide.  Action  of  Frost.  Trans- 
port of  Soil  by  Rain  and  Running  Water.  Action  of  Worms. 
Approximate  Analysis  of  Soils.  Properties  of  Clay  and  Sand. 
Chemical  Constituents  of  Soils.     Soils  and  Subsoils. 

Before  we  take  up  the  question  of  the  composition 
of  the  soil,  it  will  be  well  to  learn  something  of  its  origin, 
and  for  this  purpose  nothing  is  so  instructive  as  a  visit 
to  a  quarry  where  a  clean  face  of  rock  can  be  seen  pass- 
ing into  soil,  on  the  surface  of  which  vegetation  may 
still  be  growing.  At  some  distance  below  the  surface 
the  rock  is  solid  and  massive ;  in  some  cases,  as  in  lime- 
stone or  sandstone,  it  will  be  stratified  or  divided  into 
parallel  layers,  which  can  be  distinguished  by  changes 
of  colour  or  texture,  even  when  there  is  no  actual 
division  along  the  planes  of  bedding ;  in  other  cases,  as 
in  granite  or  basalt,  the  rock  will  be  massive  and  show 
no  sign  of  splitting  into  layers.  The  colour  of  the  rock 
will  be  pretty  uniform,  and  at  depth  it  will  often  be 
some  shade  of  dark  green  or  blue  or  grey,  though  of 
course  there  are  numerous  exceptions,  such  as  red  or 
yellow  sandstones,  white  or  yellow  limestones,  reddish 
granites,  etc.  Following  the  rock  upwards  to  the  surface, 
there  comes  a  point  when  the  rock  begins  to  show  signs 
of  decay ;  the  divisions  that  traverse  the  rock,  whether 
bedding  planes  or  joints,  become  somewhat  larger  and 

78 


Fig.  1 6.— Formation  of  a  Sedentary  Soil. 


[Face  page  78. 


CHAP,  v.]  SEDENTARY  SOILS  79 

are  discoloured,  as  though  water  has  been  oozing  along 
them  and  had  caused  a  certain  amount  of  rusting  and 
rotting.  Higher  still,  the  rock  is  obviously  disintegrated  ; 
the  large  fragments,  which  still  are  arranged  in  keeping 
with  the  structure  of  the  unaltered  rock  below,  are 
separated  by  layers  of  loose  sand  and  stones,  or  by  clay, 
and  the  proportion  of  this  loose  material  increases  the 
nearer  one  gets  to  the  surface.  Finally  the  rock  either 
wholly  disappears,  or  becomes  represented  only  by 
stones  in  the  loose  material,  which  we  may  now  call 
the  subsoil.  A  few  inches  below  the  surface,  this 
subsoil  gives  place  to  the  soil  proper,  distinguished  as  a 
rule  by  its  darker  colour  and  its  admixture  with 
vegetable  matter  from  the  plants  that  have  been  grow- 
ing upon  the  surface. 

Such  a  soil  passing  by  insensible  degrees  into  the 
rock  below  is  called  a  "  sedentary  soil,"  because  it  has 
grown  upon  the  spot ;  but  in  some  quarries  and  pits  we 
shall  not  meet  with  the  sequence  described  above,  but 
one  in  which  the  surface  material  turning  into  soil  is 
obviously  quite  distinct  from  the  fundamental  rock  below, 
a  sharp  line  of  distinction  marking  the  change  from  one 
to  the  other.  Such  cases  will  be  considered  later  ;  it  now 
remains  to  examine  into  the  causes  which  have  brought 
about  the  degradation  of  rock  into  soil.  Water  obvi- 
ously plays  a  considerable  part  in  the  process ;  even  the 
most  solid  rock  is  traversed  by  certain  planes  of  weak- 
ness— bedding  planes  or  joints,  along  which  minute 
fissures  water  slowly  percolates,  and,  as  we  have  observed 
in  the  quarry,  begins  to  degrade  and  discolour  the  edges 
of  the  cracks.  In  pure  water  itself  few  of  the  minerals 
constituting  rocks  are  soluble ;  but  the  rain,  as  it  soaks 
down  through  the  upper  layer  of  soil  pervaded  by  plants' 
roots,  dissolves  some  of  the  carbon  dioxide  that  is  there 
being  produced,  and  then  becomes  a  much  more  effective 


8o  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

solvent.  Such  water,  for  example,  will  dissolve  carbonate 
of  lime,  hence  limestone  and  chalk  rocks  waste  away 
under  the  weather,  and  leave  nothing  behind  but  the 
2  or  3  per  cent,  of  fine  red  clay,  which  is  the  only 
other  material  making  up  the  rock.  Wherever  a  railway 
cutting  traverses  chalk  or  limestone,  the  surface  of  the 
rock  will  be  seen  to  be  let  down  in  places  into  deep  and 
tapering  pipes ;  these  represent  lines  of  soakage,  along 
which  the  surface  water  has  been  continually  travelling 
to  a  fissure  below,  until  the  rock  within  the  influence  of 
this  gradual  current  has  all  been  dissolved  away.  Not 
only  does  carbonate  of  lime  occur  in  chalk  and  limestone 
in  a  more  or  less  pure  state,  but  it  is  found  in  other 
rocks  acting  as  a  cement  to  grains  of  sand,  etc.,  and  as  it 
dissolves  in  the  percolating  rain-water  the  rock  disinte- 
grates. Many  of  the  complex  minerals  which  make  up 
the  bulk  of  the  older  primitive  and  volcanic  rocks,  are 
also  decomposed  by  water  charged  with  carbon  dioxide ; 
for  instance,  the  felspar,  which  is  one  of  the  fundamental 
minerals  in  granite  and  in  all  the  basaltic  rocks,  breaks 
down  to  clay  under  its  influence.  In  Cornwall  the 
surface  of  the  granite  is  covered  with  a  white  clay  thus 
formed,  in  which  the  still  undecomposed  grains  of  quartz 
and  mica,  the  other  minerals  making  up  the  granite, 
are  lying  unaltered  ;  the  basalts  and  trap  rocks  similarly 
yield  a  clay,  which  is  red  because  of  the  oxides  of  iron 
which  are  simultaneously  formed  as  the  felspar  decays 
into  clay.  Iron  compounds  are  very  generally  diffused 
among  the  rocks ;  the  greens  and  browns  and  blacks  of 
the  unweathered  rocks  are  due  to  compounds  of  iron, 
which  all  break  up  and  go  into  solution  under  the 
action  of  water  and  carbon  dioxide,  eventually  becoming 
oxidised  into  something  akin  to  iron  rust,  and  this 
oxidised  rusty  substance  is  the  cause  of  the  yellows, 
browns,  and  reds  which  predominate  in  soils.     Perco- 


v.]  THE  WORK  OF  FROST  8i 

lating  water,  then,  acts  both  by  pure  solution  and  by 
starting  more  drastic  chemical  changes  by  the  help  of 
the  carbon  dioxide  it  carries ;  its  work  is  often  assisted 
by  the  penetration  of  roots  into  the  cracks  and  fissures. 
The  roots,  both  of  the  larger  forms  of  vegetation  and  of 
trees,  will  travel  for  astonishing  distances  and  will 
squeeze  themselves  in  the  pursuit  of  water  into  the 
closest  of  cracks ;  as  they  grow  they  exert  considerable 
pressure  to  widen  the  cracks,  and  when  they  die  and 
decay  they  leave  behind  a  channel  for  percolation. 

The  main  agency,  however,  in  splitting  rocks  and 
reducing  them  to  the  state  of  soil  is  frost,  or  rather 
water  in  the  act  of  freezing.  It  is  well  known  that 
water  expands  about  one-tenth  of  its  bulk  in  freezing, 
and  that  it  will  exert  enormous  pressure  on  anything 
that  tends  to  prevent  such  solidification  and  expansion. 
Hence  the  bursting  of  water-pipes,  full  bottles,  and  the 
like  during  a  severe  frost ;  though,  as  the  bursting  only 
becomes  apparent  when  the  water  thaws  again,  it  is  often 
supposed  that  the  thaw  does  the  bursting.  Hence  also 
the  constant  fracture  of  stones  or  bricks  exposed  to  the 
weather  during  winter;  they  become  saturated  with 
water,  and  when  this  water  tries  to  expand  within  them 
they  must  split  to  relieve  the  pressure,  thus  developing 
whatever  lines  of  weakness  there  were  within.  The 
examination  of  the  face  of  an  old  quarry  or  even  an  old 
wall  of  brick  or  stone  after  a  frost  will  show  the  ground 
at  the  foot  of  the  face  strewn  with  blocks  and  fragments 
which  have  been  wedged  off  by  the  expansion  of  the 
water  within  the  stone  or  brick.  We  talk  of  the 
pulverising  action  of  frost  on  the  clods  of  a  stiff  clay  soil ; 
frost  is  equally  if  less  actively  pulverising  to  the  toughest 
and  most  closely  grained  of  rocks.  There  are  other 
minor  agencies  assisting,  but  in  the  main,  water,  roots, 
and  frost  are  the  tools  which  are  always  at  work  reducing 

F 


82  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

rocks  to  soil,  covering  the  bare  surface  of  the  former 
wherever  it  may  be  exposed  with  a  slowly  formed 
coating  of  its  own  debris.  If  the  formation  of  soil 
ended  at  this  stage  every  rock  would  be  covered  by  its 
own  appropriate  "sedentary"  soil,  but  as  soon  as  the 
soil  is  formed  it  begins  to  move.  The  wind  and  the 
rain  are  always  moving  soil  downhill ;  the  wind  perhaps 
may  seem  a  trivial  agency,  but  in  rainless  or  even  semi- 
arid  climates  there  are  practically  no  other  transporting 
agencies  at  work,  and  we  find  vast  tracts  covered  with 
fine,  uniform,  wind-blown  soil.  The  work  of  the  rain  is 
most  apparent  when  it  has  been  long  continued  enough 
to  bring  the  rivers  out  in  flood ;  high  up  the  country 
every  little  gutter,  every  ditch  that  bounds  the  arable 
land,  is  charged  with  turbid,  hurrying  water,  which  is 
washing  unseen  quantities  of  coarser  mud  and  sand 
along  its  course.  Finally  the  rivers  themselves  are 
charged  with  sediment,  as  we  can  see  by  filling  a  large 
bottle  with  the  water  from  one  that  is  running  in  flood 
and  allowing  it  to  settle  ;  along  their  beds  also,  gravel 
and  stones  of  all  sizes  are  being  pushed.  Wherever  the 
velocity  of  the  current  is  reduced  some  of  the  transported 
material  will  be  deposited,  hence  as  the  streams  descend 
from  the  heights  into  flatter  country  they  discharge  the 
coarsest  part  of  the  material  they  have  washed  from  the 
rocks  above,  and  so  give  rise  to  the  broad  expanses  of 
gravel,  sand,  silt,  etc.,  which  underlie  the  water  meadows 
bordering  the  lower  levels  of  all  rivers.  Much  of  the 
suspended  mud  and  clay,  however,  washes  away  to  sea, 
and  is  thrown  down  when  the  velocity  of  the  stream  is 
entirely  checked  by  the  open  water,  where  also  the  salts 
in  the  sea-water  exert  their  precipitating  action  upon 
the  fine  particles ;  in  consequence,  beds  of  newly  sorted 
soil  tend  to  accumulate  in  the  shallow  waters  just  off 
shore,  and  even  to  block  up  the  river  mouth  unless  the 


V  ]  DRIFT  SOILS  83 

tides  and  currents  scour  it  free.  These  deposits  made 
by  the  river  give  us  a  clue  to  the  origin  of  that  other 
arrangement  of  rock  and  soil  that  we  have  noted  as 
sometimes  prevailing,  when  a  clean  surface  of  the 
underlying  rock  is  covered  by  gravel  or  sand  or  clay  of  a 
totally  different  character,  without  any  of  the  gradual 
transition  that  marks  the  passage  of  a  rock  into  a 
sedentary  soil.  The  layer  which  thus  rests  upon  a  rock 
out  of  which  it  has  obviously  not  been  formed  is  never 
solid  rock,  but  is  an  assemblage  of  divided  material  that 
is  generally  pretty  uniform  in  character,  wholly  gravel 
or  wholly  sand,  or  entirely  made  up  of  a  uniform  silt 
like  a  brick  earth.  The  uniformity  is  due  to  the  fact 
that  the  material  has  been  transported  to  its  present 
position  by  running  water  and  has  undergone  a  sorting 
in  the  process.  Gravels  must  have  been  laid  down  in 
the  bed  of  the  stream,  even  though  the  stream  no 
longer  runs  in  that  spot,  but  perhaps  in  a  course  at  a 
lower  level  in  the  valley.  In  such  cases  the  gravel  is 
generally  the  remains  of  a  much  larger  tract  of  gravel 
which  filled  the  valley  to  a  higher  level  at  a  time  when 
the  stream  ran  in  far  greater  volume.  Similarly  the 
sand  must  also  have  been  deposited  where  the  current 
still  ran  pretty  sharply,  whereas  the  fine-grained  and 
uniform  silts,  such  as  constitute  the  brick  earth  patches, 
were  either  deposited  in  quiet  backwaters  and  laybyes, 
or  were  dropped  by  slow  degrees  on  the  river  meadows 
in  flood  time,  just  as  similar  stuff  is  accumulating  to-day 
in  the  same  sort  of  places. 

In  addition  to  these  non-sedentary  soils  that  have 
been  moved  and  redeposited  by  running  water  or 
by  wind,  over  considerable  parts  of  the  country, 
especially  in  the  north  and  the  midlands  of  Great 
Britain,  and  for  a  hundred  miles  or  more  south  of 
the  Great   Lakes   in   the  United  States,   we   find   the 


84  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

fundamental  rocks  covered,  often  to  a  considerable 
depth,  with  disintegrated  rock  that  has  been  moved  into 
its  present  position  by  the  action  of  ice  in  the  imme- 
diately previous  geological  epoch,  when  a  glacial  climate 
prevailed.  These  glacial  drifts,  as  they  are  called,  may 
be  mainly  clay  or  sand,  but  usually  they  are  considerably 
mixed  with  fragments  of  rock  of  all  kinds  and  sizes — 
fragments  which  give  clear  evidence  of  the  actions  they 
have  been  subjected  to  among  the  moving  ice  by  the 
way  they  have  been  rounded  off  and  scored  with 
scratches  and  furrows.  These  glacial  drifts  form  an 
exception  to  what  has  already  been  said  as  to  the 
general  uniformity  of  the  transported  soil  material. 

All  soils  derived  from  material  that  has  not  grown 
up  from  the  rock  below,  but  has  been  moved  by  water, 
wind,  or  ice,  are  known  as  "  soils  of  transport "  or  "  drift 
soils,"  in  contradistinction  to  the  sedentary  soils  first 
discussed.  Some  idea  of  the  constant  state  of  motion  in 
which  soils  still  exist  may  be  obtained  by  considering 
the  surface  of  an  arable  field  on  such  soils  as  the  chalk, 
where  there  are  a  good  many  stones.  The  stones  are 
always  much  more  abundant  upon  the  top  than  in  the 
layers  below;  they  may  not  be  very  prominent  when 
the  surface  is  turned  over  by  the  plough,  but  they 
work  up  again ;  they  may  even  be  picked  off  to  sell 
for  road-making,  yet  in  a  few  years  they  seem  as 
numerous  as  ever,  so  that  the  older  farmers  and 
labourers  declare  they  "grow."  The  clue  to  their 
abundance  upon  the  surface  may  be  obtained  by 
looking  at  any  garden  bed  where  it  happens  to  receive 
the  drip  from  the  eaves  of  a  building ;  the  constant 
downpour  of  water  washes  all  the  soil  away,  and  leaves 
the  ground  covered  with  small  stones.  The  same  action 
goes  on  in  the  stony  mixture  constituting  the  soil  of  the 
arable   field ;    with   every   rainfall  there  will  be   some 


Fig.  17.— Section  showing  Glacial  Drift  lying  on  Magnesian 
Limestone. 


[Face page  84. 


v.]  ACTION  OF  WORMS  85 

rearrangement  of  the  particles,  some  washing-down  of 
the  finest  material,  until  the  large  fragments  which  are 
unmoved  by  the  rain  are  left  on  the  surface. 

Nor  are  such  inanimate  agencies  as  the  wind  and 
rain  alone  effective  in  moving  soil ;  animals  play  their 
part,  even  ants  and  moles  are  capable  in  time  of 
bringing  about  considerable  shifting  of  fine  soil  from 
below  to  the  surface.  That  some  such  action  does  go 
on  may  be  concluded  from  the  fact  that  even  when  the 
soil  is  stony  the  surface  of  an  old  pasture  is  quite  devoid 
of  stones,  to  find  which  it  may  be  necessary  to  dig  down 
for  a  foot  or  two.  This  characteristic  of  old  grass  land 
is  chiefly  due  to  the  action  of  worms,  which  are  always 
swallowing  the  finest  soil  particles,  and  then  ejecting 
them  as  wormcasts  on  the  surface.  At  first  sight  such 
an  action  seems  trivial,  but  one  has  only  to  collect  and 
weigh  on  some  autumn  morning  the  wormcasts  from  a 
square  yard  of  ground  to  find  what  a  considerable 
shifting  of  earth  has  taken  place.  Darwin  has  shown 
that  from  this  cause  alone  large  stones,  and  even  old 
buildings  of  the  time  of  the  Romans  and  later,  have 
been  completely  buried ;  he  found  that  in  one  case 
burnt  marl,  spread  on  the  surface  of  grass  land,  had  been 
buried  3  inches  in  fifteen  years,  and  in  another  case  that 
a  layer  of  chalk  had  sunk  7  inches  in  twenty-nine  years. 
A  hole  cut  in  an  old  grass  field  will  often  show  at  some 
little  depth  layers  of  chalk  or  ashes  or  burnt  soil,  each 
layer  representing  material  which  had  been  spread  on 
the  surface  years  before.  Soils  of  transport  are  formed 
in  exactly  the  same  way  as  sedentary  soils,  they  have 
merely  travelled  a  little  farther  and  been  subjected  to  a 
certain  amount  of  sorting ;  in  all  cases  we  have  to  look 
upon  soil  as  the  product  of  the  weathering — the  dis- 
integration and  decay — of  certain  fundamental  minerals 
out  of  which  the  primitive  rocks  and  the  earth's  crust 


86  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

are  built  up.  Let  us  now  see  what  these  products  are. 
It  will  first  be  necessary  to  obtain  a  certain  number  of 
typical  samples  of  soil ;  properly  they  should  be  taken 
with  an  auger  or  other  soil-sampling  tool,  but  for  the 
simple  examination  that  follows  they  may  be  obtained 
by  digging  a  hole,  2  feet  deep,  with  a  smooth  vertical 
face,  and  taking  off  from  that  face  uniform  slices  down 
to  9  inches  for  the  soil,  and  from  9  to  18  inches  for 
the  subsoil.  Samples  should  thus  be  obtained  of  both 
soil  and  subsoil,  from  a  sand,  a  clay,  and  an  alluvial 
pasture,  also  of  the  surface  soil  only  from  chalky  and 
peaty  land.  The  samples  should  be  spread  out  to  dry 
naturally  on  trays  or  sheets  of  paper,  the  lumps  being 
gently  crumbled  by  hand  before  they  get  quite  dry; 
this  being  most  necessary  with  the  clay  soils,  which  can 
only  be  broken  down  if  they  are  caught  at  just  the  right 
stage  of  moisture.  When  air-dried,  they  must  be  passed 
through  a  sieve  having  round  holes,  \  inch  or  3  mm.  in 
diameter,  the  lumps  being  crushed  in  a  mortar  with  a 
wooden  pestle  which  is  not  hard  enough  to  break  up  the 
stones.  The  material  passing  the  sieve  forms  the  fine 
earth  required  for  further  examination ;  the  stones  are 
cast  away,  though  they  should  first  be  washed  in  order 
to  see  of  what  they  are  composed.  Now  take  a  small 
quantity  of  the  fine  earth  of  one  of  the  soils  in  a  little 
dish,  and  heat  it  over  a  lamp,  gently  at  first  and  then 
more  strongly;  at  the  outset  you  will  notice  that  it 
begins  to  lose  water  although  it  was  apparently  dry  to 
start  with,  then  as  it  becomes  hotter  it  smokes  and 
gives  off  a  smell  of  burning  vegetable  matter ;  finally,  if 
heating  is  continued  for  some  time,  the  soil  will  take  a 
bright  brick-red  colour  and  undergo  no  further  change. 
From  this  we  may  conclude  that  soils  contain  water  and 
a  certain  amount  of  organic  matter  or  humus — the 
debris  of  previous  vegetation ;  it  is  most  instructive  to 


v.]  COMPOSITION  OF  SOILS  87 

determine  how  much  of  each  of  these  constituents  is 
present  in  the  examples  that  have  been  selected.    Weigh 
out  a  series  of  basins,  add  about  10  grammes  of  soil  to 
each,  and  weigh  again ;  put  the  basins  in  the  water  oven 
for  a  few  hours,  and  reweigh  to  find  the  loss  of  moisture ; 
finally,  ignite  for  two  hours  over  a  lamp   and  make  a 
last  weighing   to   obtain  the  loss    on    ignition,  which 
includes  a  little  combined  water  as  well  as  the  organic 
matter.     Considerable  differences  will  reveal  themselves 
(see  Table  XL);  the  moisture  that  is  retained  by  the 
undried  soil  will  vary  from  about  2  per  cent,  in  the 
sandy  subsoil  to  10  per  cent,  or  more  in  the  peaty  soil ; 
the  loss  on  ignition  will  similarly  vary  from  2  to  6,  and 
may  even  run  up  30  per  cent,  in  the  peaty  soil.     We 
can   see  clearly   enough   that  the  humus  present  is  a 
compound  of  carbon,  from  the  manner  in  which  it  chars 
and  blackens  on   heating.     The   humus   also  contains 
nitrogen,  as  may  be  seen  by  heating  a  little  more  of 
the   soil   mixed  with  soda-lime,  whereupon  the  smell 
of  ammonia,  which  at  once  becomes  palpable,  may  be 
taken  as  evidence  of  the  presence  of  combined  nitrogen 
in   the   humus,  just   as  in   the   plant  (p.    3).     We   can 
further  show  that  humus,  or  rather  the  humic  acid  which 
can  be  set  free  by  the  action  of  acids  on  the  humus,  is 
soluble  in  alkalis  like  ammonia  or  caustic  soda.     Take 
about  20  grammes  of  the  peaty  or  alluvial   soil,  and 
cover  it  with  dilute  (5  per  cent.)  hydrochloric  acid  for 
half  an  hour,  then  pour  off  the  acid  and  get  rid  of  it 
entirely    by   one   or    two   washings   with   pure  water. 
Finally,  pour  on  the  soil  five  or  six  times  its  bulk  of  a 
weak  solution  of  ammonia  and  shake  up  from  time  to 
time ;  a  deep  brown  or  even  black  solution  will  result, 
and  the  remaining  soil  will  be  considerably  bleached  by 
the   removal  thus   effected    of  the  greater   part  of  its 
organic  matter.     We  may  conclude  that  humus  is  one 


88  THE  ORIGIN  AND  NATURE  OE  SOILS    [chap. 

of  the  regular  constituents  of  soil,  and  is  the  result  of 
the  decay  of  the  vegetation  that  previously  lived  on  the 
soil ;  it  is  a  dark  brown  or  black  compound  of  carbon 
containing  also  nitrogen,  and  is  to  some  considerable 
extent  soluble  in  dilute  alkalis.  It  is  one  of  the  chief 
colouring  matters  of  soils,  among  the  particles  of  which 
it  also  acts  to  some  degree  as  a  cement. 

It  is  now  necessary  to  make  a  rough  mechanical 
analysis  of  the  mineral  part  of  a  soil;  lo  grammes  are 
weighed  out  into  a  dish,  a  little  water  is  added,  and  the 
contents  of  the  dish  are  rubbed  up  into  a  paste  by  means 
of  a  pestle  made  by  sticking  a  rubber  bung  on  to  the 
end  of  a  glass  rod.  A  beaker  about  8  cm.  in  diameter 
and  9  cm.  high  is  now  clearly  marked  on  the  side  at  a 
point  7-5  cm.  from  the  bottom,  and  the  pulped-up  soil  is 
washed  into  this  beaker  through  a  small  sieve  made  of 
brass- wire  cloth  of  lOO  meshes  to  the  inch.  The  sieve 
and  its  contents  are  put  in  the  oven ;  when  dry,  the 
material  on  the  sieve  is  weighed  and  counted  as  "  coarse 
sand."  The  dirty  liquid  in  the  beaker  is  now  made  up 
to  the  7-5  cm.  mark  with  more  water,  given  a  good  stir, 
and  left  to  stand  for  exactly  twelve  and  a  half  minutes, 
at  which  moment  the  upper  turbid  liquid  is  steadily  but 
quickly  poured  off  from  the  sediment  below  into  a  large 
jar.  The  sediment  is  now  rubbed  up  afresh  with  more 
water,  the  water  is  brought  to  the  mark,  stirred,  left  to 
stand  for  twelve  and  a  half  minutes,  and  poured  off  as 
before  into  the  same  jar ;  all  the  processes  being  repeated 
half  a  dozen  times  or  more,  until  at  last  there  is  nothing 
left  floating  in  the  clear  water  at  the  end  of  the  twelve 
and  a  half  minutes'  wait.  The  sediment  remaining  in 
the  beaker  is  dried  and  weighed ;  the  contents  of  the 
large  jar  can  also  be  evaporated  down  and  weighed,  but 
as  this  constitutes  rather  a  long  operation  it  will  be 
sufficient  to  evaporate  a  little  in  order  to  examine  the 


v.] 


MECHANICAL  ANALYSIS  OF  SOILS 


89 


character  of  the  deposit,  and  to  arrive  at  its  weight  by 
taking  the  difference  between  the  original  10  grammes 
and  the  sum  of  the  two  fractions  already  weighed, 
together  with  the  moisture  which  we  have  previously 
found  the  soil  to  contain.  By  this  process,  the  results  of 
which   are   set   out   in   Table   XI.,  the  soil  has  been 

Table  XI.— Approximate  Mechanical  Analysis  of  Various 
Soils. 


Sand. 

Clay. 

Alluvial. 

Chalk. 

Peat. 

Sou. 

Subsoil. 

Soil. 

Subsoil. 

Sou. 

SubfloU. 

Sou. 

Soil. 

Moisture     . 

2.4 

1-7 

4.4 

5-5 

2-4 

4.6 

2-4 

8.4 

Organic  matter    . 

40 

3-0 

6.4 

4.4 

14-3 

8-1 

6.9 

32-8 

Coarse  Sand 

10.3 

9.7 

.^•P 

0.8 

O-I 

00 

20-2 

26.6 

Fine  Sand  . 

670 

68.6 

30-1 

160 

53-3 

47-5 

21.9 

14-2 

Clay  and  Silt      . 

16.3 

I7-0 

55-2 

73-3 

29.7 

39-8 

9.6* 

18.0 

*  This  soil  also  contained  89  per  cent,  of  carbonate  of  Ume. 

divided  into  three  fractions — the  coarse  one  caught  by 
the  sieve,  which  casual  examination  and  feel  shows  to  be 
sand  ;  the  finer  one  in  the  beaker,  which  is  still  gritty  to 
the  touch  and  is  seen  under  the  lens  to  be  made  of 
rounded  particles,  so  that  it  may  be  called  fine  sand ; 
and  the  finest  stuff  of  all  that  remained  suspended  in 
the  water  poured  into  the  jar,  material  which  is  so  fine 
that  some  of  it  will  remain  floating  for  days  though  left 
quite  undisturbed.  When  we  have  collected  a  portion 
by  evaporation  or  long  settlement  it  will  feel  quite 
smooth  and  greasy  between  the  fingers,  and  only  under 
the  higher  powers  of  the  microscope  will  it  show  the 
particles  of  which  it  is  composed.  It  is  clay,  but  not  so 
wholly  clay  as  a  finer  separation  would  obtain,  being  in 
fact  a  mixture  of  what  the  soil  analyst  would  call  clay 
and  fine  silt,  but  still  for  working  purposes  a  very  pure 
clay.     Notice  particularly  that  when  it  dries  it  forms  a 


90  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

tough  cake  which  requires  some  force  to  break,  whereas 
both  the  coarse  and  fine  sand  fall  to  a  powder  as  soon  as 
they  are  dry.  If  we  make  this  partial  separation  for  all 
the  examples  of  soil  we  shall  find  the  greater  part  of 
each  of  them  to  be  composed  of  sand  and  clay  in  varying 
proportions ;  however,  even  the  coarsest  soil  is  not  free 
from  clay,  while  even  the  heavy  clay  soil  probably  contain 
50  per  cent,  of  the  sand  fractions. 

Before  examining  the  nature  of  the  sand  and  clay 
fractions  there  is  still  one  other  regular  constituent  of 
soil  which  must  be  identified,  and  this  we  may  do  by 
putting  a  few  grammes  of  each  soil  in  a  dish  and 
covering  it  with  dilute  hydrochloric  acid.  In  several 
cases,  especially  with  the  chalky  soil,  there  will  be  a 
visible  effervescence,  due  to  the  escape  of  carbon  dioxide 
from  carbonate  of  lime  in  the  soil,  which  is  being  decom- 
posed by  the  hydrochloric  acid.  Even  the  soils  which  do 
not  effervesce  probably  contain  some  carbonate  of  lime, 
small  amounts  of  which  (less  than  i  per  cent,  or  so) 
show  no  visible  evolution  of  carbon  dioxide  gas,  because 
it  dissolves  in  the  liquid  as  fast  as  it  is  set  free.  The 
peaty  soil  is  pretty  sure  to  show  no  effervescence,  and  as 
a  rule  this  is  due  to  the  entire  absence  of  carbonate  of 
lime  from  such  soils.  A  further  test  which  should  be 
applied  is  to  put  a  little  of  each  soil  in  a  wettish  condition 
on  a  strip  of  blue  litmus  paper;  the  peaty  soil,  and 
perhaps  some  of  the  others,  will  show  themselves  after 
standing  to  be  acid,  and  from  acid  soils  carbonate  of 
lime  is  absent  except  in  accidental  fragments.  These 
four  substances — humus,  sand,  clay,  carbonate  of  lime, 
constitute  the  solid  matter  of  all  soils ;  but  only  in  the 
exceptional  cases  of  peats  on  the  one  hand  or  chalky 
soils  on  the  other  do  the  sand  and  clay  fail  to  pre- 
dominate greatly  over  the  other  two  constituents. 

To  study  the  properties  of  sand  any  ordinary  clean 


v.]  PROPERTIES  OF  CLA  Y  91 

sand  will  suffice,  while  for  the  clay  a  piece  of  pure  tile 
or  brick  clay  before  it  has  been  mixed  with  sand  is 
required,  or  better  still,  a  sample  of  modelling  clay ;  some 
will  be  wanted  in  its  natural  moist  condition,  while  a 
little  should  be  dried  and  roughly  powdered.  Begin  by 
taking  some  moist  clay  and  some  wetted  sand  ;  the  clay 
can  be  moulded  into  shape  and  will  hold  together  even 
when  squeezed  into  thin  sheets,  the  sand  possesses  little 
or  no  cohesion.  On  drying,  the  sand  falls  to  pieces,  the 
clay  retains  its  shape  and  becomes  hard.  Mould  a  little 
brick  of  the  clay,  6  to  7  inches  long  and  about  i  inch 
square,  make  two  marks  on  the  face  exactly  5  inches 
apart ;  put  the  clay  aside  to  dry,  and  then  measure  the 
distance  between  the  marks  ;  the  clay  shrinks  in  drying. 
Clay  is  plastic  and  coherent  when  wet,  retains  its  shape 
on  drying,  but  shrinks  and  becomes  very  hard,  none  of 
which  properties  are  shared  by  the  sand.  Take  three 
funnels,  plug  their  stems  loosely  with  cotton-wool,  and 
weigh  out  into  one  50  grammes  of  sand,  into  each  of  the 
others  50  grammes  of  the  dry  powdered  clay ;  stand  each 
above  a  beaker,  and  pour  on  to  the  soils  100  c.c.  of  water. 
On  to  one  of  the  clay  soils,  however,  pour  a  little  water 
to  begin  with  and  make  it  into  a  paste  with  the  clay, 
adding  water  little  by  little  until  all  the  clay  is  pulped 
up  and  then  pouring  on  what  remains.  Now  note  both 
how  much  water  runs  through  and  how  long  it  takes  to 
drain  in  each  case.  The  water  runs  through  the  sand  in 
a  few  minutes,  and  when  measured  the  sand  will  probably 
not  have  retained  more  than  i  o  c.c.  of  the  water.  Through 
the  powdered  clay  the  water  may  take  an  hour  or  more  to 
pass,  and  when  the  drainage  is  complete  the  clay  may 
have  retained  almost  its  own  weight  of  water.  In  the 
third  funnel,  where  the  clay  has  been  pulped  up  with 
water  or  "puddled,"  no  water  at  all  will  have  passed. 
As  compared  with  clay,  the  sand  passes  water  quickly 


92  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

and  retains  but   a   small   proportion   when   thoroughly 
drained  ;  puddled  clay  is  quite  impervious  to  water. 

We  may  make  one  more  experiment  with  clay : 
Work  up  a  few  grammes  of  clay  with  a  gallon  or  so  of 
distilled  or  rain  water,  and  divide  the  turbid  water  that 
results  among  several  jars.  To  one  add  a  few  grains  of 
salt,  to  another  a  few  cubic  centimetres  of  lime  water, 
to  a  third  a  little  sulphate  of  lime  dissolved  in  water,  to 
a  fourth  a  few  drops  of  ammonia,  to  a  fifth  a  few  drops 
of  acid,  and  leave  a  control  without  addition ;  put  them 
all  aside  to  settle  down  undisturbed.  The  liquid  to 
which  nothing  has  been  added  will  take  a  long  time  to 
clear,  with  the  ammonia  it  will  take  longer  still,  days  or 
even  weeks.  The  acid,  on  the  contrary,  causes  the 
solution  to  clear  in  a  comparatively  short  time  ;  the  lime 
water  and  the  sulphate  of  lime  also  induce  clearing, 
though  not  so  rapidly,  and  the  salt  is  still  less  effective. 
The  appearance  of  the  quickly  clearing  liquids  gives  us 
some  clue  to  what  takes  place ;  the  acid  and  the  salts, 
especially  salts  of  lime,  induce  the  very  fine  particles  of 
which  the  clay  is  composed  to  clot  together  and  coagulate, 
or  flocculate,  as  we  shall  prefer  to  call  it.  Since  each  of 
the  compound  particles  thus  formed  must  be  heavier 
than  the  units  of  which  it  is  composed,  it  falls  to  the 
bottom  of  the  liquid  more  quickly,  and  generally  behaves 
as  though  it  were  something  of  the  order  of  a  grain  of 
sand.  Clay  particles  will  also  flocculate  if  left  to  them- 
selves and  exposed  to  changes  of  temperature,  wettings 
and  dryings,  freezings  and  thawings,  and  this  fact  has  a 
very  important  bearing  on  the  working  of  clay  soils. 
On  the  other  hand,  any  handling  or  knocking  about  of 
the  clay  soil  when  in  a  wet  condition  breaks  down  the 
flocculation  and  sets  all  the  fine  particles  free  once 
more,  making  the  clay  more  characteristically  clay  than 
ever. 


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v.]  CHEMICAL  CONSTITUENTS  OF  SOILS  93 

But  though  all  soils  are  built  up  of  sand  and  clay, 
humus  and  carbonate  of  lime,  these  materials  only 
give  rise  to  its  physical  structure ;  they  account  for 
the  manner  in  which  the  soil  works  and  its  relations  to 
water,  but  in  themselves  they  have  nothing  to  do  with 
feeding  the  plant.  In  order  to  get  information  on  that 
aspect  of  the  soil  it  is  necessary  to  try  a  new  experi- 
ment. Take  a  series  of  funnels,  plugged  as  before 
loosely  with  cotton-wool,  and  pack  each  with  a  different 
sample  of  soil,  then  pour  on  to  each  a  small  quantity  of 
hot  water  and  let  it  percolate  through,  an  operation 
which  will  be  hastened  if  each  funnel  can  be  led  into  a 
flask  communicating  with  a  filter  pump.  Take  the 
liquid  which  comes  through,  filter  if  necessary,  and 
evaporate  it  to  dryness  in  a  clean  basin — a  small 
residue  will  be  left  representing  the  material  in  the  soil 
which  was  soluble  in  water.  One  test  only  need  be 
applied  to  it ;  pour  on  to  the  residue  a  solution  of 
diphenylamine  in  sulphuric  acid  and  an  intense  blue 
colour  will  appear,  indicating  the  presence  of  nitrates  in 
the  extract  from  the  soil.  In  this  nitrate,  which  is 
present  in  all  soils,  we  have  a  substance  which,  as  we 
have  learnt  before,  will  serve  as  food  for  plants,  supply- 
ing them  with  the  combined  nitrogen  that  is  necessary 
for  their  existence.  Ammonia,  the  other  nitrogen 
compound  which  plants  can  utilise,  may  also  be  detected 
in  soil,  but  in  such  small  quantities  that  a  delicate  test  is 
necessary.  The  small  proportion  is  due  to  the  fact  that 
ammonia  in  the  soil  is  constantly  being  converted  into 
nitrates  by  a  process  which  will  be  explained  later. 
Indeed  the  nitrates  themselves  are  only  present  in  the 
soil  in  very  small  amounts,  from  two  to  twenty  parts 
per  million  of  a  dry  soil ;  how  minute  a  fraction  of  the 
whole  soil  does  the  soluble  portion  constitute,  may  be 
guessed  from  the  residue  left  in  the  dishes  after  evapora- 


94  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

tion  of  the  soil  extract.  It  is,  however,  not  quite 
sufficient  to  consider  the  soil  materials  that  are  soluble 
in  water  as  alone  capable  of  feeding  the  plant ;  we  have 
already  seen  reason  to  suppose  that  the  carbon  dioxide 
excreted  by  the  plant's  roots  and  also  present  in  the 
soil  from  other  sources,  adds  greatly  to  the  solvent 
action  of  the  water,  enabling  it  to  attack  substances 
which  are  usually  regarded  as  insoluble  in  water.  In 
consequence,  we  have  come  to  regard  any  substance  in 
the  soil  that  is  soluble  in  acids  as  a  potential  plant  food 
which  may,  however  slowly  and  in  whatever  small  quan- 
tities, be  brought  into  solution  and  so  reach  the  plant. 

If,  then,  we  attack  a  soil  with  strong  hydrochloric 
acid,  which  we  may  be  sure  will  dissolve  everything  that 
is  likely  to  reach  the  plant  within  the  next  century, 
we  shall  find  some  5  to  1 5  per  cent,  of  the  soil  going 
into  solution,  but  by  far  the  greater  part  of  this  consists 
of  oxides  of  iron  and  alumina,  to  which  (except  for  a 
trace  of  iron)  the  plant  is  indifferent.  If  we  analyse  the 
soluble  material  we  shall  find  exactly  the  elements 
we  have  previously  ascertained  to  be  common 
to  all  plants,  i.e.^  chlorine,  phosphoric  acid, 
sulphur,  and  some  silica  among  non  -  metals, 
with  soda,  potash,  magnesia,  lime,  iron,  and  alu- 
mina in  excess ;  but  the  elements  that  are  necessary 
to  the  plant  rarely  make  up  i  per  cent,  of  even  the 
most  fertile  soils.  This  i  per  cent,  however,  is  all- 
important,  constituting  as  it  does  a  vast  reservoir  of 
potential  plant  food.  Nor  is  the  quantity  so  small  as 
might  at  first  sight  appear ;  a  cubic  foot  of  soil  in  situ 
weighs  from  90  to  no  lb.,  clays  being  the  lighter  and 
coarse  sands  the  heaviest  of  all ;  at  Rothamsted,  again, 
where  the  soil  is  a  clay  loam,  the  layer  of  soil,  an  acre  in 
extent  and  9  inches  deep,  weighs  about  two  and  a  half 
million  pounds,  exclusive  of  stones,  i.e.  rather  more  than 


v.]  SOILS  AND  SUBSOILS  95 

a  thousand  tons.  If,  then,  a  soil  possesses  as  little  as 
one-tenth  per  cent  of  nitrogen,  it  still  contains  about  a 
ton  of  nitrogen  per  acre  down  to  the  depth  of  9  inches 
only;  yet,  as  we  have  seen  earlier  (Table  VI.),  an 
ordinary  crop  does  not  remove  from  the  soil  more  than 
50  to  100  lb.  of  nitrogen  per  acre.  There  is  thus  material 
for  fifty  or  more  full  crops  in  the  poorest  soils ;  but 
these  questions  will  be  discussed  more  in  detail  in  a 
later  chapter. 

We  may  now  return  to  the  rough  soil  analyses  we 
have  made,  to  see  what  light  they  throw  on  the  relative 
nature  of  the  soils  and  subsoils.  For  purposes  of 
illustration.  Table  XII.  (page  96)  gives  more  accurate 
and  detailed  analyses  of  such  a  series,  in  which  the 
soils  have  been  split  up  into  a  larger  number  of  frac- 
tions than  was  attempted  in  our  rough  analysis ;  each  of 
the  fractions  in  the  first  case  included  two  of  the  frac- 
tions into  which  the  soil  is  separated  in  the  full  analysis. 

Looking  down  this  table  we  first  notice  that  there  is 
comparatively  little  humus  in  the  subsoils ;  the  humus  is 
almost  wholly  confined  to  the  9  inches  in  which  the 
roots  are  numerous.  Exceptions  to  this  rule  are 
furnished  by  the  alluvial  and  sometimes  by  the  peaty 
soils  if  the  soil  has  been  taken  from  the  top  of  a  peat 
deposit  of  any  depth,  because  peat  is  nothing  more  than 
an  accumulation  of  former  vegetation  in  a  more  or  less 
pure  state.  The  alluvial  subsoils  always  show  a  con- 
siderable amount  of  humus,  because  down  to  their 
lowest  depths  they  consist  of  old  soil  which  has  been 
washed  off  the  surface  into  the  rivers  in  flood  time,  and 
redeposited  lower  down  the  river.  Only  when  the 
alluvial  subsoil  consists  of  coarse-grained  gravel  or  sand 
will  it  be  poor  in  humus ;  these  large  particles  naturally 
carry  no  humus  after  rubbing  together  for  a  time  in 
running  water.     As  regards  carbonate  of  lime,  soil  and 


96 


THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 


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v.]  SOILS  AND  SUBSOILS  97 

subsoil  are  very  much  alike,  the  soil  being  generally  a 
little  the  poorer,  because  of  the  constant  solution  that 
is  going  on  in  the  surface  soil.  On  the  other  hand,  the 
surface  soil  has  sometimes  been  so  regularly  dressed 
with  chalk  or  lime  that  it  shows  a  higher  percentage 
of  carbonate  of  lime  than  the  subsoil.  Soils  on  chalk 
or  limestone  formations  are  often  very  shallow,  so  that 
the  subsoil  becomes  nearly  pure  carbonate  of  lime, 
wherever  a  subsoil  can  be  said  to  exist.  As  regards 
sands  and  clay,  in  the  heavier  soils — i.e,^  the  true  clays, 
the  heavy  clay  loams,  and  most  alluvial  soils — the  surface 
soil  is  more  coarsely  grained  than  the  subsoil ;  it  contains 
more  sand  and  less  clay.  This  difference  is  due  to  the 
constant  action  of  the  rain  washing  down  the  finest 
particles,  especially  when  the  soil  is  loose  after  cultiva- 
tion. When  the  rain  beats  upon  the  rough  surface  of 
the  soil,  some  of  the  very  finest  particles  become  washed 
off  and  suspended  in  the  water;  sometimes  they  are 
filtered  out  again  by  the  soil  at  a  lower  level,  sometimes 
they  are  carried  away  into  the  ditches ;  in  either  case 
the  coarser-grained  material  is  left  behind  on  the  top. 
The  subsoil,  however,  of  a  sandy  soil  possesses  much  the 
same  composition  as  the  soil ;  in  this  case  there  is  both 
less  fine  material  to  wash  down,  and  also  the  whole  soil 
settles  together  when  beating  rain  falls,  instead  of  leav- 
ing the  hollow  spaces  which  occur  in  a  clay  soil  for  a  long 
time  after  it  has  been  worked.  As  to  the  comparisons 
between  different  kinds  of  soils,  we  find  that  all  the 
heavy  working  soils  (really  the  lightest  per  cubic  foot) 
are  made  up  of  the  finer  particles,  and  contain  but  little 
sand  ;  especially  do  they  contain  very  little  of  the  coarsest 
sand,  which  tends  to  keep  a  soil  open  and  friable.  They 
are  distinguished  by  a  high  proportion  of  clay,  though 
of  what  the  analyst  properly  calls  clay — material  made 
up  of  particles  less  than  one  hundred  and  twenty-five 

G 


98  THE  ORIGIN  AND  NATURE  OF  SOILS     [chap. 

thousandth  of  an  inch  in  diameter  (o-ooo2  mm.),  there 
is  rarely  as  much  as  30  per  cent.  In  some  very  wet  and 
heavy  working  soils,  of  which  No.  8  is  an  example,  the 
percentage  of  true  clay  is  not  especially  great,  but  it  is 
associated  with  high  percentages  of  the  fractions  next  in 
fineness — the  fine  silt  and  the  silt — and  these,  in  the 
absence  of  coarse  sand,  are  almost  as  retentive  of  water 
and  clay-like  as  clay  itself.  Such  a  soil  does  not  shrink 
so  much  on  drying,  but  it  lies  wet  for  a  long  time ;  it  is 
excessively  greasy  and  sticky  after  rain,  and  when 
worked  down  to  a  fine  tilth  will  run  together  after  rain 
and  cake  on  the  top  in  a  fashion  special  to  itself.  The 
difficulty  of  working  all  heavy  soils  is  much  mitigated  if 
they  contain  an  appreciable  quantity  of  carbonate  of  lime, 
because  that  substance  constantly  dissolves  in  the  soil 
water  which  has  gathered  carbon  dioxide  from  the 
decaying  humus,  and  thus  forms  a  solution  of  bicarbonate 
of  lime  which  keeps  the  clay  particles  flocculated.  On 
some  formations  marls  occur  containing  a  considerable 
proportion  of  very  fine-grained  carbonate  of  lime  mixed 
with  clay ;  such  soils  are  very  sticky  and  work  heavily, 
but  they  crumble  readily  when  dry,  and  never  lie  so  wet 
as  the  true  clay  soils.  Lastly,  the  sandy  soils  vary  very 
much  in  character ;  some  sandy  soils,  in  which  the  fine 
sand  and  the  silt  are  the  predominant  fractions,  are 
amongst  the  most  valuable  of  soils  for  tillage  purposes. 
They  are  light,  and  so  are  cheaply  worked ;  no  time  is 
lost,  because  they  are  fit  for  the  plough  almost  immedi- 
ately after  rain,  and  yet  for  all  the  quickness  with  which 
they  dry,  they  do  not  lose  their  power  of  supporting  the 
crop  with  water  during  a  drought.  Such  soils  may 
contain  no  more  than  10  per  cent,  of  clay,  but  owe  their 
good  qualities  to  the  general  predominance  of  fractions 
of  medium  coarseness.  The  really  poor  sands,  which 
are  often  so  light  (in  the  farmer's  sense)  as  never  to  have 


v.]  BARREN  SOILS  99 

been  brought  into  cultivation  but  remain  in  open 
common  and  waste,  may  still  contain  6  to  10  per  cent, 
of  clay,  but  the  sand  fractions  and  particularly  the  coarse 
sand,  constitute  something  like  three-quarters  of  the 
whole  soil.  Such  soils  can  retain  no  water  for  the 
service  of  the  crops  upon  them,  nor  for  reasons  that  will 
be  seen  later  can  they  retain  any  plant  food :  they  are 
poor  in  the  chemical  sense,  but  barren  chiefly  because 
of  their  dryness,  even  with  the  evenly  distributed  rain- 
fall of  Great  Britain. 


CHAPTER  VI 
CULTIVATION   AND  THE  MOVEMENTS  OF  SOIL  WATER 

Nature  of  the  Film  of  Liquid  surrounding  Wet  Particles  of  Soil. 
Retention  and  Movements  of  Water  due  to  Surface  Tension. 
Percolation.  Rise  of  Subsoil  Water  by  Capillarity.  Value  of 
Autumn  Ploughing.  Effect  of  Spring  Cultivations.  Cooling 
of  the  Land  by  Evaporation.  Effect  of  Hoeing  and  Rolling 
upon  the  Temperature  and  Water-content  of  the  Soil.  Dry 
Farming  in  Semi-arid  Regions.  Drainage  and  the  Tempera- 
ture of  Soils.     Spring  Frosts.     Early  and  Late  Soils. 

Somewhat  earlier  in  the  book  an  experiment  was 
described  (p.  91)  which  had  for  its  object  the  deter- 
mination of  the  relative  powers  of  different  soils  to 
retain  water,  though  the  water  was  given  every 
opportunity  of  draining  away.  It  will  now  be  necessary 
to  ask  how  the  soils  effect  this  retention  of  water, 
because  on  this  depend  many  of  the  properties  of  soils 
which  are  important  to  the  farmer.  It  is  not  easy  to 
observe  what  is  going  on  in  the  soil  itself,  because  the 
particles  are  so  small,  but  we  can  obtain  a  convenient 
illustration  by  means  of  a  model  on  a  large  scale. 
Procure  a  dozen  or  so  large  beads  and  thread  them  on 
three  or  four  strings  from  a  stick,  so  as  to  form  a  little 
rectangle  of  beads  hanging  vertically  and  all  in  contact ; 
this  may  be  taken  to  represent  an  imaginary  section  of 
the  soil  greatly  magnified.  Now  dip  the  beads  in  oil 
and  lift  them  out  to  drain,  or  better  still,  dip  them  in 
melted  wax  for  a  few  moments,  so  that  after  lifting  them 


Fig.  19. — Photograph  illustrating  Liquid  Film 
ROUND  Soil  Particles. 


[Face  page  100. 


CHAP.  VI.]  THE  WATER  FILM  loi 

out  the  wax  will  drain  away  for  a  short  time  before 
setting  to  a  solid  state.  Oil  or  wax  is  only  used  because 
it  exaggerates  and  renders  more  perceptible  the  results 
which  would  be  equally  given  by  water ;  the  appearances 
presented  are  shown  in  the  photograph.  It  will  be 
found  that  all  the  beads  are  covered  with  a  skin  of  oil 
or  wax,  just  as  they  would  have  been  "wetted'*  had 
they  been  dipped  in  water,  but  this  skin  is  thinnest  on 
the  upper  row  of  beads,  and  gets  perceptibly  thicker 
on  each  lower  row.  Moreover,  wherever  two  beads  are 
in  contact  there  is  a  much  thicker  layer  of  liquid  in  the 
angle  joining  the  two  surfaces,  though  again  the 
accumulations  are  greater  the  lower  the  row;  finally, 
the  lowest  row  of  beads  carry  beneath  them  large  drops 
which  are  just  not  big  enough  to  fall  off.  The  whole 
set  of  beads  must  be  considered  as  enclosed  within  a  film 
of  oil,  which  we  may  regard  not  so  much  as  sticking  to 
the  material  of  the  beads  but  as  drawn  over  them, 
because  the  outer  surface  of  the  film  acts  like  an 
elastic  skin— so  that  we  might  compare  the  arrange- 
ment to  a  set  of  balls  tied  up  in  a  stretched  sheet  of 
thin  indiarubber.  This  stretched  skin  on  the  surface 
of  a  liquid  is  always  pulling  inward  and  trying  to  shrink 
into  the  smallest  possible  area ;  for  this  reason  drops  of 
water  assume  a  round  shape  whenever  they  reach  such 
a  small  size  that  their  weight  becomes  less  effective  than 
the  elastic  pull  of  the  film,  or  the  surface  tension  as  we 
shall  call  it.  To  take  another  illustration  :  the  hairs  of  a 
dry  paint-brush  stand  out  to  form  a  round  end  ;  wet 
them,  and  they  are  dragged  together  to  a  point  by  the 
surface  tension  of  the  film  of  water  trying  to  reduce 
itself  to  the  smallest  possible  area.  It  is  not  merely  the 
contact  with  the  water  which  causes  the  pointed  end, 
because  when  the  brush  is  dipped  below  the  surface  it 
retains  the  rounded  shape  it  possesses  when  dry,  the 


I02  THE  MOVEMENTS  OF  SOIL  WATER      [chap. 

point  is  only  in  evidence  when  the  brush  is  partially  wet. 
Water  or  other  liquids  cling  to  solid  bodies  which  they 
''  wet,"  because  they  enclose  them  within  the  elastic  film 
formed  by  their  surface,  and  as  the  liquid  inside  the 
film  is  free  to  move,  the  film  must  be  equally  stretched 
at  every  point  when  equilibrium  has  been  attained  and 
the  liquid  is  at  rest  When  the  shape  of  the  wetted 
body  is  irregular,  the  film  that  encloses  it  will  always 
adjust  itself  to  develop  its  minimum  of  surface.  Coming 
back  to  the  beads  in  oil  or  wax,  the  film  on  the  upper 
surface  is  stretched  to  the  thinnest  extent  that  is 
consistent  with  not  breaking,  because  all  the  liquid 
inside  that  does  not  form  the  film  itself  is  free  to 
move,  and  sinks  by  gravity  as  low  as  possible.  On  the 
lowest  row  the  film  is  at  its  thickest,  all  the  pull  of  the 
elastic  skin  being  employed  in  holding  up  the  weight  of 
liquid  inside  the  skin.  In  the  angles  between  the  beads 
the  layer  is  thicker,  because  the  skin  is,  as  it  were, 
stretched  across  a  corner  in  order  to  reduce  its  area  to 
a  minimum.  With  this  idea  of  a  stretched  outside  skin 
in  our  minds,  let  us  imagine  what  would  happen  if 
liquid  were  added  or  taken  away  from  any  point. 
Suppose  liquid  is  added  at  the  top,  the  skin  becomes 
at  once  more  stretched,  and  tries  to  readjust  matters  by 
pressing  the  liquid  down  and  establishing  an  equal 
stretching  all  over,  and  in  this  downward  pressure  it  is 
aided  by  gravity.  Once  the  liquid  has  arrived  at  the 
lower  layer,  the  skin  there  has  to  carry  an  increased 
weight,  too  much  for  its  elasticity,  so  that  the  skin  breaks 
and  a  drop  falls.  It  is  in  this  way  that  water  percolates 
downwards  in  consolidated  soil  where  there  are  no 
actual  channels  through  which  to  flow ;  it  is  handed  on 
from  particle  to  particle,  until  at  last  it  reaches  an  open 
space  into  which  a  drop  can  form  and  break  off.  This 
open  space  may  be  a  drain-pipe,  which  then  begins  to 


VI.]  SVRPACn  TENSION  I03 

run.  It  has  often  been  thought  difficult  to  explain  how 
drains  come  to  have  free  water  in  them  at  all,  being 
themselves  composed  of  porous  material  which  clings  to 
water  like  the  fine-grained  soil  with  which  it  is  in 
contact ;  but  when  any  of  these  fine  particles  gets  so 
overcharged  with  water  that  the  elastic  skin  is  stretched 
beyond  its  holding  power,  the  skin  will  rupture  and 
detach  a  drop  of  free  water  into  any  open  space  that 
may  be  present.  Returning  now  to  our  original 
illustration  of  the  set  of  beads  dipped  in  oil,  what  will 
happen  when  liquid  is  taken  away  from  the  top  ?  The 
skin  is  reduced  in  thickness,  its  stretching  and  therefore 
its  pressure  on  the  liquid  inside  is  reduced,  so  that  a 
rise  of  liquid  takes  place  from  below  until  the  tension  is 
once  more  equalised  all  over.  Thus  water  can  rise  in  a 
soil  from  the  lower  wet  layers  in  order  to  replace  that 
which  has  been  taken  away  by  evaporation  at  the 
surface,  but  it  is  necessary  that  there  shall  be  a  con- 
tinuous film  of  water  from  the  evaporating  surface  to 
the  wet  layer  below.  If  the  evaporation  continues  until 
the  films  become  thinner  and  thinner  and  more  and 
more  stretched  ;  at  last  the  stretching  becomes  too  much 
for  the  elasticity  of  the  material,  the  film  ruptures  and 
shrinks  up  to  a  smaller  surface  with  a  relieved  internal 
strain.  Similarly,  if  liquid  is  taken  away  at  the  side 
there  will  be  thinner  films  created  at  that  point  with 
reduced  stretching,  which  exerts  a  pull  on  the  liquid 
until  the  tension  of  the  film  is  once  more  equalised  all 
over.  Water  will  move  horizontally  from  'a  wetter 
to  a  drier  area,  just  as  it  can  move  upwards  against 
gravity,  or  still  more  readily  downwards  with  gravity ; 
as  long  as  the  film  of  water  round  the  particles  is 
continuous,  this  motion  in  order  to  restore  equilibrium 
can  take  place.  This  property  of  water  and  other 
liquids  to   generate  a   kind   of  stretched   skin   on   the 


104  THE  MOVEMENTS  OF  SOIL  WATER      [chap. 

surface  is  known  as  surface  tension ;  it  is  only  another 
way  of  looking  at  the  property  which  is  sometimes 
called  capillarity,  the  power  of  liquids  to  move  upwards, 
downwards,  or  sideways,  along,  or  into  any  system  of 
fine  tubes.  By  means  of  a  few  glass  tubes  drawn  out 
fine  and  some  coloured  water,  one  may  easily  see  that 
water  will  rise  in  such  tubes  above  the  level  outside, 
and  will  rise  farther  the  finer  the  tube  in  which  the 
experiment  is  tried.  When  one  corner  of  a  sponge  or  a 
towel  is  dipped  into  water,  the  whole  soon  becomes 
wetted,  and  the  behaviour  of  the  water  may  either  be 
regarded  as  a  creation  of  a  stretched  film  from  fibre  to 
fibre,  or  a  motion  of  the  water  along  the  fine  channels 
formed  by  the  closeness  of  the  fibres.  In  the  same  way 
water  may  be  conceived  to  move  in  the  soil  along  the 
fine  channels  formed  between  particles  which  are  touch- 
ing at  their  salient  points ;  it  is  quite  easy  to  show  that 
soil  is  never  packed  solid,  there  is  always  about  40  per 
cent,  of  free  pore-space  between  the  particles,  like  the 
spaces  which  must  exist  between  the  spheres  making 
up  a  heap  of  shot,  however  closely  they  are  packed 
together.  But  just  as  we  have  seen  that,  for  any  motion 
of  water  to  take  place  in  the  soil,  the  film  of  water  must 
be  continuous  from  particle  to  particle,  so  from  this 
other  point  of  view  there  must  be  no  gaps  in  the  soil 
destroying  the  continuity  of  the  fine  tubes,  because 
liquids  can  move  but  a  small  distance  along  a  wide  tube. 
In  interpreting  the  relations  between  water  and  the  soil 
particles,  either  way  of  looking  at  things — capillarity 
or  surface  tension  —  may  be  employed ;  perhaps  the 
stretched  film  is  the  more  useful,  because  it  fixes 
attention  on  the  fact  that  the  surface  exposed  by  the 
soil  particles  is  their  active  and  effective  part  in  all  such 
actions.  It  is  easy  to  understand  that  the  more 
extensive    the    total    surface    possessed    by    the    soil 


L, 


vn.%*iv*. . 


if 


hTaXer 


J^fy-ouj^ 


Fig.  20.— Diagram  illustrating  Capillary  Rise 
OF  Liquids. 


\Face  page  104. 


VI.]  SURFACE  OF  SOIL  PARTICLES  105 

particles,  the  bigger  area  there  is  to  which  water,  or 
anything  else  wetting  the  surface,  will  cling;  further- 
more, the  smaller  the  particles  into  which  a  given 
weight  of  soil  is  divided,  the  greater  the  total  area  of 
surface.  Imagine  a  little  cube,  i  inch  on  the  side, 
divided  by  three  cuts  into  eight  cubes,  each  \  inch  on  the 
side ;  the  surface  exposed  by  the  solid  material  has  been 
doubled,  because  for  one  cube  with  six  faces,  each  an  inch 
square,  there  are  now  eight  cubes,  each  with  ^\>l  faces  a 
quarter  of  a  square  inch  in  area,  or  12  square  inches  in 
all.  In  consequence,  a  really  fine-grained  soil  possesses 
an  enormous  area  and  a  corresponding  absorbing  power ; 
it  has  been  calculated  in  various  ways  that  the  particles 
contained  in  a  cubic  foot  of  an  average  heavy  soil 
possess  surfaces  making  up  a  total  area  of  about  an 
acre,  while  in  a  clay  soil  this  is  increased  to  about  4 
acres.  All  surface-tension  actions  are,  therefore,  more 
powerful  in  clay  and  similarly  fine-grained  soils;  they 
absorb  a  greater  amount  of  water,  and  hold  on  to  it 
more  tightly.  The  rate  of  motion  of  water,  however,  in 
such  soils  is  slower,  whether  it  be  the  downward  move- 
ment due  to  gravity  and  surface  tension  together  which 
we  call  percolation,  or  the  upward  movement  due  to 
surface  tension  alone  when  the  surface  is  losing  water. 
The  slowness  of  the  movements  is  due  to  extreme 
fineness  of  the  channels  between  the  minute  clay 
particles;  the  water  in  these  channels  cannot  move 
freely,  because  it  is  never  removed  from  the  drag  caused 
by  the  attracting  surfaces  of  the  solid.  Skin  friction  is 
always  at  work,  and  there  is  never  anything  approaching 
free  flow. 

Applying  these  principles  to  the  soils  themselves,  we 
see  that  in  a  coarse-grained  sandy  soil  percolation  will 
always  be  rapid,  because  the  mere  motion  of  water  by 
gravity  in  the  large  spaces  between  the  particles  will  be 


io6  THE  MOVEMENTS  OF  SOIL  WATER      [chap. 

little  impeded  by  skin  friction.  Drainage  will  therefore 
be  good,  but  since  the  soil  particles  possess  so  small  a 
surface  that  can  be  wetted,  the  soil  will  therefore  retain 
very  little  moisture,  sometimes  no  more  than  lo  per  cent, 
when  fully  saturated  but  drained.  Similarly,  the 
particles  being  large  the  water-films  round  them  are 
not  extensive,  and  will  not  possess  much  power  of  lifting 
water  against  gravity ;  when  evaporation  is  going  on,  the 
water  will  rise  quickly  because  it  is  little  impeded,  but 
to  no  great  distance.  The  capillary  tubes  formed  by 
the  interspaces  between  the  sand  grains  are  large,  so 
motion  up  them  is  quick,  but  for  only  a  short  distance. 
In  the  opposite  way,  in  a  very  finely  grained  soil  like  a 
clay,  percolation  will  be  slow,  because  the  downward 
motion  is  hindered  by  the  clinging  of  the  extensive 
surface  to  the  water,  and  by  the  fineness  of  the  inter- 
spaces through  which  it  has  to  ooze.  This  extensive 
surface,  however,  retains  a  considerable  proportion  of 
water,  up  to  40  per  cent,  when  drainage  is  over  but 
losses  by  evaporation  have  not  begun.  Again,  although 
the  pull  exerted  by  so  great  a  surface  is  enormous,  it  is 
very  ineffective  in  drawing  up  water  rapidly,  because  of 
the  drag  exerted  by  the  particles  on  the  water  with 
which  they  are  in  such  close  contact.  When  the  clay 
has  been  tempered  and  kneaded  so  as  to  reduce  it  to* 
its  maximum  of  fine  division  and  also  to  pack  the 
particles  together  as  closely  as  possible,  water  will  not 
move  through  it  at  all,  because  the  water  particles  are 
held  so  tightly  by  the  clay  particles  with  which  they  are 
in  contact  that  to  all  intents  and  purposes  they  cannot 
move  at  all.  For  practical  purposes  the  most  valuable 
soils  are  those  which  are  fine-grained  enough  to  possess 
considerable  surface,  capable  of  retaining  water  against 
drainage,  and  also  of  lifting  it  from  the  wetter  layers 
below    when    the    proportion    has    been    reduced    by 


VI.]  WATER  REQUIRED  BY  CROPS  107 

evaporation  at  the  surface,  but  yet  are  sufficiently  coarse 
to  admit  of  the  reasonably  free  movements  of  water 
either  downwards  or  upwards.  There  is,  in  fact,  a  happy 
medium  of  grain  which  has  a  large  and  effective  surface 
that  is  yet  not  too  large  to  shut  down  the  movement  of 
the  water.  Such  soils  are  found  among  some  of  the 
loams  and  fine  sands  in  which  predominate  those 
fractions  which  we  have  previously  designated  as  fine 
sand  and  silts. 

We  have  already  discussed  the  amount  of  water 
required  by  the  growing  plant,  and  have  found  that 
taking  the  experimental  figure  of  200  to  300  pounds 
of  water  transpired  from  each  pound  of  dry  matter 
elaborated,  then  the  usual  crops  grown  in  this  country 
will  evaporate  from  6  to  10  inches  of  rain,  and  even 
more  in  the  case  of  a  heavy  crop  of  mangolds.  Con- 
sidering that  this  growth  is  all  made  during  a  portion  of 
the  year  only,  that  evaporation  from  the  bare  soil  will 
also  dissipate  much  of  the  rainfall,  a  great  deal  of  which 
also  runs  off  the  surface  or  percolates  so  quickly  into 
the  subsoil  as  to  lead  to  its  permanent  loss  to  the  land 
by  drainage,  we  can  easily  understand  that  where  the 
annual  rainfall  is  only  30  inches  or  so,  as  is  the  case 
over  the  greater  part  of  England,  the  crops  are  often 
likely  to  suffer  for  want  of  water.  Humid  as  is  the 
English  climate,  the  magnitude  of  the  crops  in  any  given 
year  is  more  often  determined  by  lack  of  water  than  by 
any  other  single  factor,  and  the  majority  of  the 
operations  of  cultivation  are  directed  towards  obtaining 
and  storing  as  large  a  water-supply  as  possible. 
Beginning  in  the  autumn,  it  is  desirable  to  get  the 
stubbles  ploughed  early  and  left  rough  for  the  winter ; 
much  more  of  the  winter  rainfall  will  be  absorbed  by 
such  a  ploughed  surface  than  by  one  which  remains  so 
firm  and  trampled  by  the  harvest  operations  that  water 


io8  THE  MOVEMENTS  OF  SOIL  WATER      [chap. 

penetrates  with  difficulty  and  largely  runs  off  to  the 
ditches.  Not  only  is  more  water  collected,  but  the 
exposure  of  the  plough  slice  to  the  weather,  to  the 
pulverising  action  of  freezings  and  thawings,  wettings 
and  dryings,  tends  to  flocculate  the  clay  particles  on 
heavy  soils,  so  that  later  they  are  easily  worked  down 
to  a  fine  tilth.  This  weathering  action  has  even  some 
effect  on  the  chemical  constituents  of  the  soil ;  the  potash 
compounds  in  particular  are  found  to  become  more 
soluble  after  a  winter's  exposure  of  the  soil  to  frost 
and  rain.  The  autumn  ploughing  is  particularly  valuable 
on  really  heavy  soils  ;  they  are  rarely  dry  enough  during 
the  winter  or  early  spring  to  be  fit  for  cultivation ; 
in  the  process  they  become  somewhat  tempered  and 
smeared  by  the  plough,  kneaded  also  by  the  treading 
of  horses  and  man,  so  that  they  would  dry  into  tough 
and  difficult  clods  were  they  not  afterwards  exposed  to  a 
succession  of  spring  frosts  to  bring  them  into  condition 
again.  One  cultivation  of  a  clay  soil  in  a  wet  state  in 
the  spring  may  easily  spoil  its  working  and  therefore  its 
productivity  for  the  rest  of  the  year,  because  there  are 
no  longer  any  frosts  to  restore  the  tilth,  whereas  a 
mistake  in  the  autumn  becomes  repaired  by  the  action 
of  the  later  frosts. 

Assuming  that  the  land  has  been  ploughed  in  the 
autumn,  it  is  then  desirable  to  plough  once  more  in 
the  spring  as  soon  as  the  land  will  carry  horses  safely — 
the  object  of  this  second  ploughing  being  to  conserve 
the  main  supply  of  water  in  the  subsoil,  and  also  to 
enable  the  layer  on  the  surface  to  become  dry  and 
warm  so  as  to  be  ready  for  the  formation  of  a  seed 
bed.  After  the  winter  rains  the  plough  slices  will  have 
become  somewhat  closely  beaten  down  and  joined  up 
once  more  into  a  compact  mass  with  the  earth  forming 
the  subsoil  below  ;  thus  the  film  of  water  on  the  particles 


VI.]  COOLING  BY  E  VAPOR  A  TION  109 

at  the  surface  is  joined  up  to  that  surrounding  the 
particles  in  the  subsoil  below,  with  the  result  that  as  fast 
as  water  is  evaporated  from  the  surface  the  losses  are 
repaired  from  the  subsoil.  This  not  only  causes  a  loss 
of  water  to  the  soil — a  loss  which  may  become  of 
importance  later — but  the  continuous  evaporation  keeps 
the  surface  moist  and  cold.  It  should  always  be  realised 
that  an  evaporating  liquid  withdraws  heat  from  the 
objects  with  which  it  is  in  contact — the  heat  being 
necessary  to  convert  the  liquid  into  vapour.  This  with- 
drawal of  heat  is  well  seen  in  the  cold  experienced  by 
the  wetted  hands  or  face  when  a  dry  east  wind  is 
blowing  and  causing  rapid  evaporation ;  it  may  be 
illustrated  experimentally  by  wrapping  the  bulb  of  a 
thermometer  in  wet  flannel  or  cotton-wool  and  whirling 
it  rapidly  round  and  round  in  the  air,  whereupon  the 
temperature  indicated  by  the  thermometer  may  be 
made  to  fall  several  degrees.  Again,  the  cooling  effect 
of  evaporation  may  be  demonstrated  by  the  cold 
experienced  when  a  little  alcohol,  ether,  or  petrol  is 
poured  on  the  hands.  Being  all  readily  volatile  liquids, 
they  withdraw  a  considerable  amount  of  heat  from  the 
hands  in  order  to  pass  into  the  state  of  vapour.  The 
spring  cultivation  of  the  previously  ploughed  land 
breaks  up  the  continuity  of  the  water  films  between  the 
plough  layer  and  the  subsoil  below,  water  thereupon 
travels  much  more  slowly  into  the  surface  layer,  which 
in  consequence  can  become  dry,  and  being  dry  can  also 
warm  up  at  an  earlier  date.  The  actual  drying  will 
probably  take  place  a  little  more  gently,  because  the 
land  is  not  forced  to  wait  until  the  parching  east  winds 
set  in ;  when  heavy  wet  land  is  dried  rapidly  under 
their  influence  a  hard,  steely  crust  is  generally  formed 
on  the  surface  which  is  difficult  to  break  down  later. 
The  early  spring  ploughing  before  the  frosts  are  quite 


no  THE  MOVEMENTS  OF  SOIL  WATER      [chap. 

ended  is  of  the  greatest  importance  in  getting  a  heavy 
clay  soil  to  break  down  to  a  mellow  crumbly  seed  bed. 
The  remaining  operations  of  early  spring  are  mostly 
directed  towards  obtaining  a  good  seed  bed  by  the  use 
of  cultivators  of  various  kinds,  and  on  heavy  soils  the 
important  thing  to  be  observed  is  never  to  work  the 
land  when  it  is  in  the  least  wet,  or  clods  of  deflocculated 
clay  must  inevitably  be  the  result,  clods  which  dry  with 
difficulty  and  then  form  hard  tough  masses  which  can 
only  be  worked  down  by  the  action  of  frost.  By 
implements  alone — cultivators,  rollers,  or  clod-crushers 
— it  is  impossible  to  reduce  the  lumps  formed  by 
smeared  and  deflocculated  clay  into  a  fine  tilth. 

But  while  the  farmer  is  reducing  his  soil  to  a  crumb,  so 
as  to  secure  a  fine  seed  bed  in  which  he  can  deposit  the 
seeds  at  an  even  depth,  he  is  also  careful  to  keep  the  soil 
consolidated,  because  if  it  becomes  loose  below  the 
surface  there  can  no  longer  be  any  continuity  of  liquid 
films  between  the  water  in  the  subsoil  and  that  round 
the  particles  in  contact  with  the  roots  of  the  young 
plant.  After  sowing,  the  land  is  generally  further 
consolidated  by  rolling,  and  in  a  dry  spring  the  rolling 
may  be  repeated  just  after  the  seeds  have  begun  to  push. 
The  effect  of  rolling  is  to  consolidate  the  whole  soil 
from  the  surface  downwards  ;  it  brings  the  soil  particles 
into  more  intimate  contact  and  assists  the  formation  of 
continuous  water  films  from  the  wet  subsoil  below.  As 
these  water  films  are  continuous  to  the  surface  there 
will  be  loss  of  water  by  evaporation,  with  a  consequent 
reduction  of  temperature  ;  but  these  disadvantages  have 
to  be  faced,  in  view  of  the  vital  importance  of  keeping 
up  the  water-supply  from  below  to  the  germinating 
seeds  or  infant  plants  in  time  of  drought.  By  experi- 
ment, we  can  find  that  land  which  has  been  rolled  is  (i) 
moister  at  the  surface  but  drier  at  9  inches  or  a  foot 


VI.]  ROLLING  AND  HOEING  in 

down  ;  (2)  cooler  at  the  surface  than  is  a  piece  of  adjoin- 
ing land  which  has  been  kept  loosely  covered  with  soil. 
Too  much  stress  cannot  be  laid  on  the  importance  of 
consolidating  the  soil  round  the  roots  of  plants, 
especially  in  their  earlier  stages  of  growth ;  although 
gardeners  are  accustomed  to  tread  over  their  beds 
before  sowing  small  seeds  like  onions,  they  often  allow 
the  soil  of  flower-beds  to  become  altogether  loose  and 
porous  down  to  considerable  depth,  simply  by  doing 
too  much  trenching  and  deep  digging. 

As  soon  as  the  plant  has  been  established  the  chief 
work  of  the  farmer  or  gardener  consists  in  maintaining 
a  loose  surface  of  the  soil  for  the  remainder  of  the  season. 
Not  only  are  the  various  processes  which  liberate  plant 
food  from  the  reserves  in  the  soil  promoted  by  a  constant 
stirring  of  the  surface,  but  above  all  things  the  stock  of 
water  in  the  soil  is  conserved  for  the  uses  of  the  plant 
alone.  A  loose  surface  tilth  soon  dries  itself,  but  as  it 
possesses  no  close  connection  with  the  layer  below  there 
is  no  continuous  water  film  from  the  water-bearing  layers 
to  the  surface  at  which  loss  is  suffered  by  evaporation. 
The  water  is  lifted  by  surface  tension  as  far  as  the  soil 
is  consolidated,  i.e.  up  to  the  roots  of  the  plant,  but  it 
cannot  get  into  the  layer  which  is  actually  exposed  to 
the  drying  wind  and  sun,  because  this  layer  is  only  lying 
loosely  on  the  water-holding  soil  with  large  gaps 
between.  The  loose,  friable  layer  of  soil  acts  as  a  screen 
on  the  soil  below,  protecting  it  from  evaporation  :  it  is  in 
fact  a  soil  mulch,  and  serves  just  the  same  purpose  as  a 
layer  of  farmyard  manure,  or  straw,  or  grass  clippings 
— any  of  the  materials  which  a  gardener  usually  calls  a 
mulch.  By  keeping  it  constantly  renewed,  the  soil 
mulch  may  be  completely  effective  in  preserving  all  the 
water  originally  present  in  the  soil  until  the  only  loss  of 
water  to  the  soil  is  that  which  the  crop  takes  and  utilises. 


112  THE  MOVEMENTS  OF  SOIL  WATER     [chap. 

An  old  gardener  was  accustomed  to  say  that  he  watered 
his  garden  with  his  hoe  ;  and  in  this  connection  it  should 
be  noted  that  a  light  watering  or  a  gentle  shower  of 
rain  during  a  drought  often  causes  more  loss  than  gain 
of  moisture  to  the  soil.  The  wetting  of  the  surface  soil 
which  ensues  may  be  only  sufficient  to  re-establish  the 
film  of  water  round  the  soil  particles  at  the  top  and  make 
that  film  continuous  with  the  permanent  film  reaching  up 
from  the  wetted  subsoil.  In  consequence,  as  soon  as 
evaporation  begins  afresh  at  the  surface,  water  is  lifted 
from  the  lower  layers  to  takes  its  place,  and  the  process 
goes  on  with  considerable  loss  to  the  stock  of  water  in 
the  soil  unless  the  continuity  of  the  film  near  the  surface 
is  ruptured  by  cultivation.  Waterings  should  be  done 
thoroughly  or  not  at  all,  and  they  should  be  followed 
up  by  hoeing  as  soon  as  the  surface  is  fit  to  work  again. 
The  fact  that  the  crop  evaporates  or  transpires  con- 
siderable quantities  of  water  has  already  been  noted,  and 
also  the  consequence  that  the  growth  of  a  crop  leaves 
the  soil  very  much  drier  than  it  was  before.  The 
converse  of  the  proportion  is  seen  in  the  fact  that  a  year 
of  bare  fallow,  in  which  no  crop  is  grown  but  the  surface 
is  kept  stirred  to  check  evaporation,  will  accumulate  a 
considerable  proportion  of  that  year's  rainfall  in  the 
subsoil  for  the  benefit  of  subsequent  crops.  In  semi-arid 
areas  of  deficient  rainfall  such  as  prevail  in  many  parts 
of  western  America  and  Canada,  in  southern  Russia, 
and  in  regions  bordering  the  Australian  deserts,  farming 
is  rendered  possible  with  a  rainfall  of  lo  to  15  inches  per 
annum  by  taking  advantage  of  bare  fallows.  It  is 
customary  to  take  only  one  crop  in  two  years,  or  two 
crops  in  three  years ;  in  the  intermediate  years  the  soil 
is  kept  ploughed  so  as  to  establish  a  surface  which  will 
both  collect  any  rain  that  falls  and  will  also  evaporate 
but  little  because  of  its  looseness.     The  chief  features  of 


VI.]  DR  Y  FARMING  1 1 3 

this  "dry  farming"  in  arid  areas  are  the  insistence  on 
the  establishment  of  a  soil  mulch — the  loose  dry  layer 
of  soil  which  will  prevent  evaporation — and  on  "  subsoil 
packing,"  as  it  is  called,  i.e.,  the  consolidation  of  the  soil 
immediately  below  the  mulch,  so  that  it  will  continuously 
and  steadily  lift  whatever  water  may  have  been  accumu- 
lated in  the  subsoil  as  far  as  the  roots  of  the  plant.  To 
generate  the  soil  mulch,  shallow,  broad-tined  cultivators 
and  harrows  are  used,  while  ring  rollers  are  used  to 
effect  the  subsoil  packing.  These  principles  of  "dry 
farming  "  have  been  before  the  mind  of  the  British  farmer 
from  time  immemorial,  especially  when  farming  upon 
some  of  the  light  sandy  and  chalky  soils  in  the  east  and 
south-east  of  England,  where  the  rainfall  is  light  and  the 
evaporation  active.  To  grow  satisfactory  root  crops 
under  these  conditions  demands  great  skill  in  the 
preparation  of  the  soil,  and  the  whole  art  of  the  farmer 
is  therefore  concentrated  on  getting  a  good  seed  bed. 
First  of  all,  if  the  soil  is  heavy  he  has  to  work  it 
with  judgment  to  get  it  to  fall  down  into  a  crumb  at  all ; 
then  he  strives  to  render  this  crumb  deep  and  uniform, 
thoroughly  consolidated,  and  in  intimate  contact  with 
the  undisturbed  subsoil  below,  but  at  the  same  time 
possessing  a  loose,  friable  surface  which  will  admit  air 
yet  protect  the  layer  in  which  the  roots  are  growing 
from  the  evaporation  that  follows  exposure  to  sun  and 
wind.  It  is  possible  to  understand  the  principles  under- 
lying the  farmer's  operations  and  aims,  but;  only 
experience  can  teach  the  moment  at  which  a  difficult 
soil  may  be  moved  with  success  so  as  to  bring  it  into  the 
desired  condition.  Such  knowledge,  however,  of  what  to 
aim  at  in  the  cultivation  of  the  soil,  of  the  best  tool  to 
take  and  the  right  opportunity  to  seize  for  each  operation, 
lies  at  the  basis  of  all  husbandry.  The  art  of  the  arable 
farmer  may  be  summed  up  as  the  proper  preparation  of 


114  THE  MOVEMENTS  OF  SOIL  WATER     [chap. 

a  seed  bed,  for  on  that  depends  the  future  of  the  crop  to 
a  far  greater  extent  than  on  the  inherent  fertility  of  the 
soil  or  the  amount  of  manure  it  receives. 

The  effect  of  the  water-content  of  the  soil  upon  its 
temperature  is  one  of  the  most  important  factors  which 
the  cultivator  of  the  soil  has  to  bear  in  mind :  it  has 
already  been  explained  that  the  evaporation  of  water 
results  in  a  considerable  lowering  of  temperature,  so 
that  the  heat  of  the  spring  sun  may  be  largely  spent  in 
drying  a  soil  without  raising  its  temperature  at  all. 
The  amount  of  heat  necessary  to  raise  the  temperature 
of  a  pound  of  water  by  one  degree  would  heat  a  pound 
of  dry  soil  by  about  seven  degrees,  whereas  it  would 
require  about  a  thousand  times  as  much  heat  to 
evaporate  a  pound  of  water  and  even  then  cause  no  rise 
of  temperature.  Growth  does  not  take  place  below  a 
temperature  of  about  41°  F.,  this  being  also  the  limit 
below  which  most  seeds  do  not  start  nor  do  the 
roots  of  a  plant  function,  hence  before  any  growth  can 
take  place  in  the  spring,  it  is  necessary  to  raise  the  soil's 
temperature  above  this  limit.  Almost  the  only  source  of 
heat  is  the  radiation  from  the  sun,  which  is  absorbed  by 
the  soil  and  converted  into  heat ;  but  to  ensure  that  the 
heat  shall  be  effective  in  raising  temperature,  evaporation, 
and  therefore  the  access  of  the  soil  moisture  to  the 
surface,  must  be  checked  by  cultivation.  Any  method 
of  diminishing  evaporation  will,  of  course,  aid  in  raising 
the  temperature  of  the  soil ;  for  example,  a  rough  surface 
to  the  ground,  wind  breaks,  hedges,  or  belts  of  trees, 
which  check  the  sweep  of  the  wind  over  the  ground,  are 
very  effective  in  early  spring  towards  getting  the  soil 
quickly  into  condition  for  maintaining  growth.  In 
many  places  the  producers  of  early  vegetables  on  light 
land  near  the  sea  have  learnt  how  desirable  it  is  to 
establish   wind   breaks   by  erecting    lines   of  thatched 


VI.]  DRAINAGE  115 

hurdles  or  temporary  reed  fences  across  their  land. 
Similarly,  stones  upon  the  surface  of  the  land  both  help 
the  ground  to  warm  up  more  quickly  and  maintain  the 
stock  of  water  later  in  the  season  when  droughts 
set  in. 

The  effects  of  drainage  upon  cultivated  land  afford 
several  illustrations  of  the  principles  underlying  the 
operations  of  cultivation.  In  the  first  place,  the  drained 
land  is  warmer  and  therefore  earlier,  because  as  the 
level  of  permanent  water  is  no  longer  near  the  surface, 
less  water  will  be  brought  up  to  be  evaporated  ;  conse- 
quently, drying  and  warming  of  the  surface  soil  can  take 
place  comparatively  quickly.  At  the  same  time  the 
temperature  of  the  air  above  the  soil  is  also  found  to  be 
raised  by  drainage.  In  the  second  place,  with  the 
lowering  of  the  water-level  in  the  soil,  air  is  introduced 
to  greater  depths,  the  root  system  of  the  crops  can 
extend  farther,  and  in  consequence  the  plant  has  a 
greater  soil  layer  to  draw  upon,  and  is  thereby  better 
able  to  resist  a  drought.  Lastly,  the  introduction  of  air 
more  deeply  with  the  soil  and  the  replacement  of  the 
old  stagnant  water  conditions  by  a  steady  movement  of 
water  through  the  soil,  results  in  the  improvement  of  its 
texture ;  the  clay  particles  are  flocculated  both  by  the 
occasional  dryings  and  by  the  salts  that  are  washed 
through ;  to  a  certain  extent  also  the  very  finest  particles 
of  all  are  washed  down  into  the  drains.  The  improve- 
ment of  the  texture  of  the  soil  by  drainage  is  of  course 
a  question  of  time,  and  only  becomes  apparent  some 
years  after  the  drains  have  been  put  in. 

Just  as  the  drainage  of  a  heavy  or  a  waterlogged  soil, 
by  reducing  the  amount  of  water  with  which  it  is 
burdened,  causes  the  soil  to  warm  up  more  quickly  in 
the  spring,  and  therefore  to  become  earlier,  so  the 
question  of  early  or  late  soils  is  largely  determined  by 


ii6  THE  MOVEMENTS  OP  SOIL  WATER      [chap. 

the  quantity  of  water  they  retain.  Only  a  light,  coarse- 
grained soil,  possessing  a  good  natural  drainage,  can  free 
itself  quickly  from  the  winter  rainfall,  and  therefore  can 
warm  up  quickly  ;  but  whether  such  a  soil  will  also  be  an 
early  one,  depends  also  upon  its  situation,  aspect,  and  a 
few  minor  factors  like  colour.  The  heating  of  a  soil 
is  practically  wholly  due  to  the  sun  ;  although,  if  a  soil  is 
made  very  rich  in  fresh  organic  matter,  like  farmyard 
manure,  there  will  be  a  little  heating  of  the  soil  owing 
to  the  rapid  decomposition  of  organic  matter  that  ensues. 
In  this  case  the  soil  has  been  to  some  degree  converted 
into  a  hot  bed ;  but  putting  this  rare  factor  aside,  the 
colour  of  the  soil  has  a  certain  influence  upon  the  com- 
pleteness with  which  the  sun's  rays  are  absorbed  and. 
converted  into  heat  within  the  soil.  Black  soils  will  be 
the  most  effective,  red  soils  come  next.  One  practical 
application  of  this  fact  lies  in  the  use  of  soot  as  a  top- 
dressing  for  wheat  in  the  early  spring ;  the  soot  contains 
a  little  ammonia  which  acts  as  manure,  it  also  checks 
the  attacks  of  the  slugs  and  small  snails  which  often  do 
damage  to  the  young  corn,  but  it  has  a  further  beneficial 
action,  because  its  colour  enables  the  soil  to  absorb  more 
heat,  enough  in  fact  to  raise  the  temperature  by  one  or 
two  degrees  on  a  sunshiny  day.  A  very  simple  experi- 
ment will  demonstrate  this  :  a  small  plot  of  ground,  a  few 
yards  square  will  suffice,  must  be  brought  into  a  state 
of  good  uniform  tilth  in  March  or  April,  and  one-half 
should  be  lightly  dusted  over  with  soot,  so  as  to  blacken 
the  surface,  without  protecting  it  by  any  such  coating  as 
would  act  as  a  mulch.  Two  ordinary  thin-stemmed 
thermometers  (which  should  be  checked  against  one 
another  beforehand),  are  then  plunged  in  the  soil,  one 
in  each  portion,  care  being  taken  to  bury  the  bulbs  at 
exactly  the  same  depth  of  3  inches  and  to  make  the 
soil  firm  round  them.     On  a  bright  day  of  continuous 


VI.]  ASPECT  117 

sunshine,  the  thermometer  below  the  blackened  soil  will 
register  from  one  to  four  degrees  higher  temperature 
than  the  other  one  beneath  the  uncoloured  soil.  Aspect 
is  also  a  matter  of  considerable  importance — a  field 
sloping  to  the  south,  not  only  receives  the  sun's  rays 
both  earlier  in  the  morning  and  later  in  the  evening, 
than  another  field  facing  north,  but  also  the  intensity  of 
the  rays  upon  a  given  area  is  greater.  Since  the  sun  is 
never  vertically  overhead  in  this  country,  the  area  that 
has  to  be  covered  and  warmed  by  a  given  beam  of  sun 
is  smaller  when  it  falls  on  ground  sloping  towards  the 


Fig.  21.— Distribution  of  Sun's  Rays  upon  Southerly 
AND  Northerly  Slope. 

beam,  ue.  southwards,  than  when  it  slopes  the  other  way. 
The  diagram.  Fig.  21,  shows  on  to  how  much  smaller  an 
area  the  sunbeam  AB  is  concentrated  when  the  area 
slopes  towards  instead  of  away  from  the  beam.  An 
early  soil  should  also  be  a  sheltered  one ;  wind  always 
increases  the  evaporation  from  the  soil,  and  therefore 
reduces  its  temperature;  the  use  of  hedges  and 
screens  of  any  kind  is  of  value  to  the  raiser  of 
early  vegetables,  provided  he  does  not  allow  them  to 
keep  the  sun  off  his  crops.  Height  above  sea-level  is 
also  a   factor   demanding  consideration ;   putting  aside 


ii8  THE  MOVEMENTS  OF  SOIL  WATER      [chap. 

local  variations,  there  is  a  steady  fall  of  average  tempera- 
ture with  increasing  elevation,  so  that  nearly  all  the  soils 
in  Britain  usually  known  as  early  and  used  for  the 
productions  of  the  first  vegetable  crops,  are  comparatively 
near  the  sea-level.  At  the  same  time,  soils  which  lie 
at  the  bottom  of  valleys  are  subject  to  late  frosts  in  the 
spring,  and  early  frosts  in  the  autumn  ;  the  frosts  always 
come  more  severely  and  more  frequently  in  the  lowest 
parts  of  the  valley,  and  often  do  not  extend  many  feet 
up  the  sides  of  the  valley.  Such  frosts  arise  in  still 
weather  and  cloudless  nights,  and  are  due  to  the  cooling 
down  of  the  earth's  surface  by  radiation  into  space,  when 
there  are  no  clouds  to  act  as  a  blanket.  If  the  atmo- 
sphere is  also  still  in  such  radiation  weather,  the  air  in 
contact  with  the  ground  becomes  chilled,  whereupon  it 
contracts  and  grows  denser.  Owing  to  its  increased 
density  it  then  begins  to  roll  downhill  like  so  much 
water  and  accumulates  at  the  bottom  of  the  valley  until 
it  is  displaced  by  the  still  colder  air  which  occurs  later 
in  the  night.  Stillness  is  necessary,  otherwise  the  cooled 
air  would  not  lie  in  the  valleys,  but  soon  become 
disseminated.  The  proximity  of  the  sea  or  other  large 
body  of  water,  is  also  a  considerable  factor  in  the  pro- 
duction of  an  early  soil ;  the  water,  because  of  its  large 
specific  heat  tends  to  keep  temperatures  equable,  and 
does  much  to  mitigate  the  untimely  spring  or  early 
autumn  frosts  which  are  so  much  dreaded  by  the  fruit 
and  vegetable  grower.  Nearly  all  the  districts  in 
which  early  potato-growing  is  practised  in  the  United 
Kingdom  are  near  the  sea,  e.g.  Jersey,  the  neighbour- 
hood of  Penzance,  Ayshire,  Kerry,  parts  of  West 
Lancashire,  and  a  few  places  on  the  seaboard  of  Kent 
and  Essex,  etc. 

A   soil   which   is   thus   early   because  of  its  coarse 
texture,  its  dryness,  the  readiness  with  which  it  can  be 


VI.]  HE  A  VY  AND  LIGHT  SOILS  1 19 

worked  after  rain,  its  aspect  and  situation,  develops 
certain  disadvantages  later  in  the  season.  It  is  apt  to 
dry  out  with  even  a  short  drought ;  it  often  reduces  the 
yield  of  ordinary  crops  by  thus  checking  the  growth  at 
critical  stages,  even  if  the  crop  is  not  forced  to  ripen  off 
too  early.  Such  soils,  again,  often  give  rise  to  very  rapid 
growth  in  the  autumn  after  the  rains ;  this  growth,  how- 
ever, is  often  soft  and  stands  very  badly  in  a  severe  winter. 
A  heavy  soil  which  is  late  to  warm  up,  for  the  same 
reason  often  holds  its  temperature  until  well  on  into 
the  autumn ;  and  crops  growing  upon  such  heavy  soils 
will  continue  their  development  far  into  the  winter. 
On  them  the  whole  course  of  development  is  slower 
and  more  uniform,  and  this  results  in  subtle  differences 
in  quality  between  the  produce  of  heavy  and  light  soils, 
which  in  the  present  state  of  our  knowledge  can  be 
better  appreciated  by  experience  than  explained  by 
science. 


CHAPTER  VII 

THE  LIVING  ORGANISMS  OF    THE  SOIL 

Formation  of  Nitrates  in  the  Soil.  Bacteria  in  the  Soil  which 
decompose  Organic  Matter.  Bacterial  Loss  of  Nitrogen  from 
the  Soil.  Formation  of  Humus.  Fixation  of  Nitrogen  by 
Bacteria  associated  with  Leguminous  Plants.  Value  of  Clover 
Crops  in  the  Rotation.  Inoculation  of  Soil.  Other  Bacteria- 
fixing  Nitrogen  in  the  Soil.  Accumulation  of  Nitrogen  in 
Virgin  Soils.     Dependence  of  Soil  Fertility  upon  Bacteria. 

So  far,  we  have  considered  the  soil  as  though  it  were 
a  merely  dead  medium  in  which  the  plant  can  fix  itself, 
and  from  which  it  can  obtain  food  by  processes  of  solution 
and  diffusion,  as  though  the  whole  behaviour  of  the  soil 
towards  the  plant  were  regulated  by  chemical  and 
physical  actions  of  the  kind  that  can  be  reproduced  in 
the  laboratory.  Yet  certain  facts  might  serve  to  suggest 
that  other  changes  have  their  seat  in  the  soil ;  for 
example,  we  have  ascertained  by  water-culture  experi- 
ments that  plants  can  only  take  in  their  necessary 
nitrogen  in  the  forms  of  nitrates  or  ammonia,  small 
amounts  of  which  are  very  general  constituents  of 
cultivated  soils.  But  though  nitrate  of  soda  and  sulphate 
of  ammonia  are  employed  as  manures,  their  purpose  is 
equally  and  more  commonly  served  by  other  compounds 
of  nitrogen,  such  as  the  animal  and  vegetable  residues 
which  are  found  in  ordinary  dung.  Both  experiment 
and  farming  experience  show  that  the  plant  will  obtain 
nitrogen  from  whichever  of  its  compounds  is  put  into 

120 


CHAP.  VII.]  NITRIFICATION  121 

the  ground,  yet  we  do  not  know  of  any  means  by  which 
these  nitrogen  compounds  can  be  converted  into  nitrates 
in  the  laboratory  except  by  most  difficult  and  involved 
processes.  The  whole  history  of  nitrates  in  the  soil  will, 
in  fact,  repay  a  little  study.  It  has  been  known  from 
time  immemorial  that  nitre  in  some  form  or  other  is  a 
product  of  the  soil,  and  in  India  the  men  of  a  whole 
caste  or  guild  make  their  living  by  gathering  nitre.  They 
scrape  away  the  surface  soil  from  time  to  time  in  the 
neighbourhood  of  the  villages  just  beyond  the  point  where 
the  drainage  finds  an  outlet,  extract  it  with  hot  water, 
and  then  after  clarification  concentrate  the  solution  thus 
obtained,  the  result  being  the  nitrate  of  potash,  nitre,  or 
saltpetre  of  commerce.  Similarly,  in  Egypt  the  native 
cultivator  knows  that  the  earth  from  deserted  villages 
— the  dust,  in  fact,  into  which  the  old  mud-built  habita- 
tions resolve  themselves — forms  a  valuable  fertiliser  and 
will  yield  nitre  when  extracted  with  water.  Again,  nitre 
used  to  be  obtained  in  Europe  from  the  earth  of  cellars, 
stable  floors,  and  other  fairly  dry  places  near  habitations  ; 
in  later  times  it  was  expressly  manufactured  by  building 
up  nitre  beds — mixtures  of  earth  with  a  certain  amount 
of  carbonate  of  lime  and  organic  matter,  over  which 
urine  and  similar  liquids  containing  compounds  of 
nitrogen  were  poured  from  time  to  time.  The  bed  was 
protected  from  the  rain,  was  kept  moist  but  not  wet,  and 
after  a  period  of  from  three  to  five  years  it  was  extracted 
with  water  and  yielded  a  solution  of  nitrate  of  lime, 
which  could  be  afterwards  converted  into  nitrate  of 
potash  for  gunpowder-making.  This  well-known  process 
of  nitrification  represents  the  fact  that  the  soil  is  able  to 
bring  about  a  change  in  the  organic  compounds  of 
nitrogen  and  convert  them  into  nitrates,  either  nitrate  of 
lime  or  nitrate  of  potash,  according  to  the  base  which 
happens  to  be  present.     Long  before  the  mechanism  of 


122  LIVING  ORGANISMS  OF  THE  SOIL       [chap. 

the  change  was  understood  men  had  learnt  that  it  was 
forwarded  by  a  proper  degree  of  moisture,  by  warmth, 
and  by  the  presence  of  a  base  Hke  carbonate  of  lime. 
Not,  however,  until  about  1 877  was  it  demonstrated  that 
the  agents  effecting  the  change  are  certain  minute 
organisms  living  in  the  soil  and  called  bacteria,  which  are 
classed  with  the  plants,  though  they  are  so  lowly  in  type 
that  they  might  almost  be  regarded  as  the  common 
meeting-ground  of  the  lines  of  plant  and  animal  descent. 
At  first  only  the  fact  was  demonstrated  that  the  change 
was  due  to  something  living,  and  this  was  done  by 
showing  that  nitrification  ceases  when  the  soil  is  kept 
permeated  by  the  vapour  of  chloroform,  which  will 
inhibit  or  destroy  living  organisms.  Again,  if  the  soil  is 
heated  to  the  temperature  of  boiling  water  it  loses  its 
power  of  forming  nitrates.  The  first  step  having  thus 
been  taken,  subsequent  investigators  cleared  up  the 
various  steps  in  the  process  and  succeeded  in  isolating 
from  the  soil  in  an  unmixed  state  two  organisms,  one  of 
which  will  change  ammonia  into  nitrite  and  the  other 
will  complete  the  oxidation  into  nitrate.  We  can 
demonstrate  their  action  by  a  few  simple  experiments ; 
an  ammoniacal  solution  that  also  contains  the  other 
elements  necessary  to  the  nutrition  of  bacteria  is  made 
up  as  follows : — 

2  grammes  Ammonium  Sulphate 
0'2  „  Potassium  Phosphate 
o-i  „  Magnesium  Sulphate 
o-i  „         Sodium  Chloride 

are  dissolved  in  i  litre  of  water,  and  a  few  drops  of 
ferric  chloride  solution  are  also  added.  One  hundred 
cubic  centimetres  of  this  solution  are  placed  in  each  of 
four  flasks  of  about  3CX)  c.c.  capacity,  the  mouths  of 
which  are  afterwards  plugged  with  cotton-wool.  The 
flasks  and  their  contents  are  then  sterilised  by  heating 


O    3J       V 

o  a    Si. 
g.S    cl 

as, 


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*^  3 
CO 
O   > 

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•—  in" 


CTJ 


^         Is- 

^         ^g- 


t3   rt 
C  -O 


VII.]  NITRIFICATION  123 

them  in  a  steamer  for  an  hour  or  so  to  the  temperature 
of  boiling  water.  One  flask  is  retained  as  a  check,  to 
another  about  a  gramme  of  fresh  soil  is  added,  to  a 
third  and  fourth  a  gramme  of  soil  each  and  half  a 
gramme  of  carbonate  of  lime.  One  of  these  latter 
flasks  is  then  heated  up  to  boiling  again  for  half  an 
hour.  The  four  flasks  are  put  aside  in  a  warm  and  dark 
place  for  a  period  of  three  weeks  or  a  month,  and  the 
liquid  in  each  is  then  tested  for  the  presence  of  nitrates 
by  taking  out  one  drop  of  it  on  a  loop  of  platinum  wire 
and  introducing  it  into  a  solution  of  diphenylamine  in 
sulphuric  acid.  The  control — the  nutrient  solution  to 
which  no  soil  has  been  added — will  show  no  nitrate ; 
sometimes  the  solution  to  which  soil  only  has  been 
added  will  also  show  none,  but  in  the  solution  to  which 
both  soil  and  carbonate  of  lime  have  been  added, 
nitrates  will  be  found  and  the  ammonia  may  have 
entirely  disappeared.  Finally,  the  fourth  flask  which 
had  been  boiled  after  the  addition  of  the  soil  will  also 
show  no  change,  only  the  unaltered  ammonia  remains 
without  nitrates.  In  this  simple  way  it  is  possible  to 
demonstrate  that  some  living  agency  is  forming  nitrates 
in  almost  all  cultivated  soils.  These  agents  are,  how- 
ever, absent  from  peaty  and  acid  soils,  and  do  not  occur 
in  subsoils  taken  below  a  depth  which  varies  with  the 
nature  of  the  soil.  Another  instructive  experiment  may 
be  made  by  filling  a  glass  tube  about  5  feet  long  and  i 
inch  in  diameter  with  fragments  of  chalk,  the  cork 
which  closes  the  bottom  of  the  tube  being  supplied  with 
one  exit-tube  to  drain  off  any  liquid,  and  another  rather 
longer  tube  turned  over  inside,  through  which  air  can  be 
admitted  to  the  interior.  The  tube  is  first  of  all  seeded 
with  the  nitrifying  bacteria  by  running  some  well-water 
through  it,  or  by  putting  a  little  soil  on  the  top  of  the 
chalk,  then  a  dilute  ammoniacal  solution  or  even  a  liquid 


124  LIVING  ORGANISMS  OP  THE  SOIL        [chap. 

like  diluted  urine  or  liquid  manure  or  sewage  is  allowed 
to  drip  very  slowly  on  to  the  top  of  the  chalk,  so  slowly 
that  the  liquid  takes  several  days  to  work  its  way  from 
piece  to  piece  of  the  chalk  before  it  finds  its  way  out  at 
the  exit.  When  the  tube  has  grown  into  good  working 
order  the  ammoniacal  liquid  will  undergo  pretty 
complete  nitrification  as  it  soaks  down  the  chalk,  and 
only  nitrates  will  be  found  leaving  the  tube,  but  the 
introduction  of  a  little  chloroform  into  the  tube  will 
soon  put  a  stop  to  the  action. 

Though  it  was  the  first  to  be  investigated,  nitrifica- 
tion is  only  one  of  the  many  changes  going  on  in  the 
soil  that  are  due  to  bacteria,  and  some  of  these  changes 
are  of  the  utmost  moment  to  the  nutrition  of  the  plant 
and  to  the  maintenance  of  the  fertility  of  the  soil.  As 
regards  changes  in  the  nitrogenous  compounds,  another 
experiment  can  be  performed  which  will  illustrate  the 
main  features  of  the  cycle  that  is  perpetually  being 
worked  through.  A  flask  is  made  up  as  before  with 
lOO  C.C.  of  nutrient  solution,  in  which,  however,  the 
sulphate  of  ammonia  is  replaced  by  an  equal  weight  of 
peptone,  this  being  a  complex  carbon  compound  of 
nitrogen  closely  related  to  flesh,  albumen,  etc.,  but  more 
convenient  in  this  connection  because  it  is  soluble.  Half 
a  gramme  of  carbonate  of  lime  is  placed  in  the  flask, 
which  is  then  sterilised  as  before  in  the  steamer ;  when 
cold,  about  i  gramme  of  soil  is  added.  The  flask  is  put 
away  in  the  dark  as  before,  and  at  the  end  of  a  week  it 
is  examined.  It  will  be  found  that  tlie  first  change  to 
take  place  has  been  putrefaction  ;  the  liquid  possesses  a 
strong  and  offensive  smell  and  is  turbid  with  swarms  of 
organisms  it  contains,  some  of  which  are  easily  visible  if 
a  little  of  the  liquid  is  placed  under  the  higher  powers 
of  the  microscope.  The  liquid  is  then  restored  to  the 
dark  for  another  fortnight  or  so,  when  the  putrefactive 


VII.]  SOIL  BACTERIA  125 

smell  will  have  disappeared  and  been  replaced  by  a 
faint  smell  of  ammonia,  the  next  step  in  the  cycle  of 
transformations.  Another  month  in  the  dark  will  show 
the  third  and,  as  a  rule,  the  final  stage  well  on  its  way — 
the  previously  formed  ammonia  is  now  being  converted 
into  nitrate.  Lastly,  the  fate  of  the  nitrate  may  be  illus- 
trated by  leaving  the  flask  in  the  light  for  a  few  weeks, 
whereupon  a  bright  green  growth  of  algae  will  appear 
from  spores  introduced  with  the  original  soil.  In 
sunlight  the  algae  will  form  a  scum  buoyed  up  by  the 
bubbles  of  oxygen  set  free,  because  the  algae  are  green 
plants  which  possess  chlorophyll  and  split  up  carbon 
dioxide  like  other  higher  plants.  After  the  algal  growth 
has  made  some  headway  the  liquid  may  be  tested,  and 
will  no  longer  show  either  nitrate  or  ammonia;  these 
substances  have  been  absorbed  by  the  growing  algae 
and  reconverted  into  proteins  by  being  drawn  into 
combination  with  the  carbon  compounds  which  the 
algae  have  assimilated  from  the  atmospheric  carbon 
dioxide.  Thus  in  the  flask  has  been  illustrated  the 
whole  cycle  of  the  breakdown  by  bacteria  and  the 
reconstruction  in  the  living  plant  of  the  protein  bodies 
which  are  fundamental  to  both  plants  and  animals. 
This  all-important  set  of  changes  is  brought  about  by 
at  least  three  distinct  sets  of  bacteria.  The  first  com- 
prises numerous  forms  or  species  of  the  so-called 
putrefactive  bacteria,  which  seize  upon  the  complex 
nitrogenous  bodies  like  the  proteins  and  break  them 
down  into  simpler  substances,  among  which  are  certain 
compounds  containing  sulphur  and  possessing  character- 
istic disagreeable  smells,  the  unpleasantness  of  the  smell 
being  probably  nothing  more  than  a  physiological 
memory  of  the  fact  that  intestinal  disorders  have  always 
followed  the  consumption  of  food  possessing  such  a  smell 
and  therefore  swarming  with  bacteria.     After  this  first 


126  LIVING  ORGANISMS  OF  THE  SOIL        [chap. 

putrefactive  breakdown  another  set  of  bacteria  takes  up 
the  resulting  nitrogenous  compounds  and  breaks  them 
down  still  further  into  ammonia,  whereupon  the  nitrifying 
bacteria  can  begin  to  exercise  their  function.  Com- 
pounds in  the  soil  are,  however,  subject  to  other  kinds 
of  change,  as  we  can  show  by  a  further  experiment.  A 
flask  of  about  lOO  c.c.  capacity  is  fitted  with  a  good  cork 
and  a  single  exit-tube  leading  downwards  and  then 
bent  up  at  the  extremity :  into  this  flask  about  lO 
grammes  of  soil  and  a  \  gramme  of  sugar  are  placed, 
then  the  flask  is  completely  filled  with  a  solution 
containing  i  per  thousand  of  potassium  nitrate.  After 
the  contents  of  the  flask  have  been  shaken  a  little  to 
get  the  sugar  dissolved,  the  cork  and  exit-tube  are 
inserted  ;  the  latter  is  completely  filled  with  water  and 
set  up  so  that  it  dips  into  water  and  is  directed  into  the 
open  end  of  an  inverted  test-tube,  also  filled  with  water. 
The  apparatus  is  then  put  away  in  a  dark,  warm  place, 
and  examined  from  time  to  time ;  after  some  days 
bubbles  of  gas  will  begin  to  appear  and  will  collect  in 
the  test-tube.  When  enough  of  the  gas  has  been 
collected  it  may  be  examined  with  a  lighted  taper,  and 
by  shaking  up  with  lime  water  it  will  prove  to  consist 
in  the  main  of  nitrogen  gas.  If  at  the  same  time  the 
liquid  remaining  in  the  flask  be  tested  it  will  no  longer 
show  any  reaction  for  nitrate;  this  salt  has  been 
decomposed  by  certain  organisms  in  the  soil  with  the 
liberation  of  nitrogen  gas.  Obviously  this  is  a  very 
wasteful  process,  because  nitrates  and  all  forms  of 
combined  nitrogen  are  valuable,  whereas  four-fifths  of 
the  atmosphere  is  made  up  of  nitrogen  gas.  Any  such 
change  in  cultivated  soils  ought  so  be  checked  as  much 
as  possible,  and  this  can  only  be  done  by  avoiding  the 
conditions  which  give  rise  to  the  decomposition.  Now 
the   essential   conditions   of  the   experiment   were   the 


VII.]  DENITRIFICA  TION  1 27 

presence  of  organic  matter  like  sugar,  which  is  easily 
oxidised,  and  the  exclusion  of  all  air,  so  that  the  soil 
bacteria  in  want  of  oxygen  are  driven  to  take  it  out  of 
the  nitrate  which  is  present.  In  nature  the  soil  does 
lose  nitrogen  because  of  bacterial  changes  of  this  kind ; 
we  call  the  process  denitrification,  but  the  losses  will 
only  be  large  when  there  is  much  easily  oxidisable 
organic  matter  present  in  the  soil,  and  when  the  soil 
gets  short  of  air  by  waterlogging  or  some  similar 
accident.  That  the  losses,  however,  may  be  consider- 
able can  be  deduced  from  one  of  the  experimental  plots 
on  the  wheat  field  at  Rothamsted,  where  the  large 
quantity  of  14  tons  per  acre  of  farmyard  manure  are 
applied  every  year  for  the  wheat  crop.  Despite  all  this 
application  of  manure  the  yield  of  wheat  has  not 
continued  to  increase ;  indeed,  for  the  last  forty  years 
it  has  been  almost  stationary,  except  for  seasonal 
fluctuations.  The  amount  of  nitrogen,  however,  that  is 
obtained  in  the  wheat  crop  is  only  about  one-quarter  of 
the  200  lb.  per  acre  that  was  applied  in  the  manure. 
Of  the  remaining  three-quarters  of  the  annual  supply 
of  nitrogen,  about  one-quarter  still  remains  in  the 
soil,  as  an  unexhausted  residue  which  will  very  slowly 
be  converted  into  plant  food.  But  one-half  of  the 
immense  amount  of  manurial  nitrogen  applied  has  been 
lost  entirely ;  it  has  been  dissipated  as  nitrogen  gas  by 
the  bacteria  in  the  soil  causing  what  we  have  called 
denitrification,  and  the  loss  has  been  so  great  because  of 
the  enormous  accumulation  of  organic  matter  "in  the 
soil.  Denitrification  causing  loss  of  soil  capital  must 
be  added,  then,  to  the  list  of  bacterial  actions  always 
going  on  in  the  soil,  or  ready  to  take  place  whenever 
the  conditions  become  favourable  to  that  particular  form 
of  activity. 

So    far,  however,  we    have    only    been  considering 


128  LIVING  ORGANISMS  OF  THE  SOIL       [chap. 

changes   brought   about   by  bacteria   in   the   nitrogen- 
ous   compounds   present   in  or   added  to  the   soil,  but 
non-nitrogenous    materials     undergo    equally     marked 
decompositions,  often  of  great  practical  importance.     To 
illustrate  the  nature  of  these  changes,  two  experiments 
may  be  performed.     A  tall  bottle,  which  has  an  inlet  at 
the  bottom,  is  filled  with  dry  soil,  mixed  with  a  small 
quantity  of  finely  powdered  sugar,  about  2  grammes  of 
sugar  per  kilogram   of  soil.     The   action   is  the  same 
with    all    carbohydrates,    but   it   will   take  place  more 
rapidly  with  sugar,  so  that  it  is  the  preferable  substance 
to  use.     Sufficient  water  is  slowly  poured  in  to  moisten 
the  soil,  then  the  bottle  is  put  aside  in  a  warm  place, 
and   once   a   day   the   gas  it  contains  is   pumped  out 
through  a  wash-bottle  containing  lime  water,  air  being 
allowed  to  enter  in  its  place.      It  will  be  found   that 
quantities  of  carbon  dioxide  are  given  off  by  the  soil, 
and  in  a  week  or  two  if  the  soil  be  washed  and  the 
solution  filtered  and  tested  no  trace  of  sugar  will  be 
detected.     It  has  all  been  oxidised  by  the  bacteria  of 
the  soil  into  carbon  dioxide,  air  of  course  having  been 
also  necessary  to  the  process.     In  the  other  experiment 
a    corked-up    flask    with    an    exit-tube,   such    as    was 
employed  in  the  denitrification  experiment,  is  required. 
The  flask  is  partly  filled  with  a  solution  of  nutrient  salts, 
such  as  was  used  for  water  cultures,  and  a  number  of 
strips  of  clean  filter  paper,  which  is  nearly  pure  cellulose, 
is  introduced  together  with  a  small  quantity  of  soil,  or 
better  still,  of  pond  mud.    The  flask  and  tube  are  filled  up 
with  water,  and  made  to  lead  into  an  inverted  test-tube 
over  water  as  before,  then  the  whole  apparatus  is  put 
aside  in  a  warm  place  in  the  dark.     It  may  be  some 
days  before  any  action  sets  in,  but  eventually  bubbles 
of  gas  will  arise  and  will  collect  in  the  test-tube,  which 
can    be    removed    for   their  examination  from  time  to 


VII.]  CELLULOSE  FERMENTATION  129 

time.     Carbon  dioxide  will  always  be  found,  but  if  that 
is  removed  by  shaking  up  the  contents  of  the  tube  with 
lime  water  or  a  little  caustic  soda,  some  other  gas  will 
also  remain,  and  this  gas  will  take  fire  if  a  light  be 
brought  to  the  mouth  of  the  test-tube.     The  inflammable 
gas  is  generally  marsh-gas,  sometimes  hydrogen,  or  a 
mixture  of  the  two.     At  the  same  time,  it  will  be  seen 
that  the  filter  paper  is  being  attacked ;  the  strips  get 
thinner  and  begin  to  break  up  in  places  ;  finally  they 
disappear    entirely,   or   give    place   to   a    dark   brown, 
structureless  mass.     On   testing  the   liquid  it   will   be 
found  to  be  acid.     In  this  type  of  fermentation,  which 
takes  place  without  any  contact  with  the  air,  cellulose 
and  other  carbohydrates  become  split  up  into  carbon 
dioxide   and   either  marsh-gas  or   hydrogen,  a  certain 
amount  of  humus  being  formed  at  the  same  time.     It 
should  be  noted  that  all  the  changes  described  in  this 
and  the  preceding  experiments  will  not  take  place  if  the 
action  of  bacteria  is  suspended   at  starting  by  intro- 
ducing chloroform  into  the  mixture,  or  by  sterilising  the 
whole  by  boiling ;  they  are  all  processes  due  to  living 
agencies,  and  in  most  cases  we  cannot  reproduce  them 
in  the  laboratory  by  purely  inanimate  means.     These 
two  modes  of  breaking  down  carbohydrates  illustrate 
processes  which  are  widespread  in    nature.     The  first, 
in  which  the   carbon   compounds   are   oxidised  in  the 
presence  of  air  to  carbon  dioxide,  represents  the  slow 
decay  which  overtakes  all  organic  matter  when  freely 
exposed.     A  leaf,  a  branch,  even  a  tree  itself,  which  falls 
to  the  ground  in  a  wood  is  eventually  dissipated  into 
gas,  and  leaves  behind  nothing  more  solid  than  the  ash 
which  would  have  resulted  if  it  had  been  burnt.     The 
process  of  aerial  decay  is,  in  fact,  neither  more  nor  less 
than  a  slow  burning  brought  about  by  living  organisms ; 
not  only  bacteria  effect  such  actions,  but  many  fungi, 

I 


I30  LIVING  ORGANISMS  OF  THE  SOIL        [chap. 

and  even  insects,  worms,  and  other  lowly  creatures  help 
to  produce  the  same  change.  The  other  change,  which 
takes  place  in  the  absence  of  air,  is  seen  when  a  branch 
of  a  tree  falls  into  a  pond  or  a  deep  ditch  and  becomes 
buried  in  the  mud.  Thus  cut  off  from  the  air  it  changes 
much  more  slowly,  but  it  does  lose  carbonic  acid,  marsh- 
gas,  and  sometimes  hydrogen.  Thereby  it  also  becomes 
gradually  black,  and  on  analysis  proves  to  be  richer  in 
carbon  than  the  original  wood,  because  it  has  been 
giving  off  in  the  gases  that  arise  from  it  more  of  its 
hydrogen  and  oxygen  than  of  its  carbon.  This  process 
of  an  anaerobic  (out  of  contact  of  air)  change  is  seen 
most  markedly  in  the  formation  of  peat  from  vegetable 
matter  on  waterlogged  land,  but  it  has  its  share  in  the 
production  of  the  humus  of  all  cultivated  soils. 

We  have  now  reviewed  the  main  types  of  change 
brought  about  by  bacteria  in  the  organic  matter  of  the 
soil ;  the  carbon  compounds  are  being  broken  down  into 
simpler  bodies  and  in  nearly  all  cases  eventually  become 
carbon  dioxide,  the  regeneration  of  which  into  the 
complex  organic  matter  of  the  carbohydrates,  etc.,  is 
effected  by  the  living  plant.  The  nitrogen  compounds 
are  also  broken  down  into  successively  simpler  bodies, 
finally  into  ammonia  and  nitrates,  which  the  living  plant 
can  again  utilise  and  reconstruct  into  the  more  complex 
protein  forms.  Some  nitrogen  also  is  always  being 
thrown  out  of  combination,  and  passed  into  the  air  in 
the  comparatively  useless  state  of  nitrogen  gas.  It  will 
be  noticed  that  we  have  seen  no  agency  at  work  to 
reconvert  nitrogen  gas  into  any  form  of  combination, 
nothing  in  fact  to  account  for  the  original  stock  of 
combined  nitrogen  in  the  world.  Plants  can  only  make 
use  of  nitrogen  when  it  is  already  combined  as  ammonia 
or  nitrates;  they  build  up  successively  more  complex 
bodies  from  these  substances,  but  not  from  nitrogen  gas 


VII.]  FIXATION  OF  NITROGEN  131 

itself.  The  soil  bacteria  we  have  been  discussing 
similarly  only  cause  the  circulation  of  nitrogen  from  one 
form  of  combination  to  another  ;  one  type  sets  nitrogen 
free,  but  none  that  we  have  yet  examined  do  anything 
to  bring  it  back  into  combination.  But  if  the  combined 
nitrogen  of  the  world  had  only  been  suffering  loss  by 
change  into  nitrogen  gas  ever  since  the  beginning  of 
things,  however  small  those  losses  may  appear  to  be,  we 
should  probably  have  arrived  at  some  evidence  of  a 
diminution  of  the  stock  and  signs  of  its  ultimate 
exhaustion.  No  indications  of  the  sort,  however,  exist, 
from  which  it  might  be  concluded  that  some  agency 
must  be  at  work  replenishing  the  stock  by  "  fixing " 
nitrogen  gas  and  bringing  it  back  into  combination. 

The  discovery  of  such  an  agency  arose  out  of  the 
long-debated  question  of  whether  our  ordinary  plants, 
which  are  always  in  contact  with  a  boundless  store  of 
nitrogen  gas,  do  not  possess  in  their  leaves  the  power 
of  thus  "  fixing "  the  nitrogen  they  require.  Many 
of  the  earlier  investigators  were  firmly  convinced  that 
plants  had  such  a  power,  arguing  from  the  difficulty 
of  understanding  how  there  could  be  combined  nitrogen 
in  the  world  from  any  other  source.  Moreover,  great 
refinement  of  experiment  is  necessary  to  demonstrate 
that  the  nitrogen  gained  by  a  plant  is  exactly  balanced 
by  the  amount  lost  by  the  soil,  hence  the  controversy 
lasted  for  a  considerable  time,  even  if  it  can  be  regarded 
as  ended  now.  However,  the  weight  of  evidence,  and 
notably  a  series  of  experiments  carried  out  by  Dr  Evan 
Pugh  of  Pennsylvania  at  Rothamsted  in  1857-8,  coupled 
with  the  experience  gained  by  field  trials  of  the  great 
value  to  the  crop  of  a  supply  of  manurial  nitrogen,  have 
led  people  ever  since  the  middle  of  last  century  to  accept 
as  a  general  principle  the  opinion  that  plants  do  not  fix 
nitrogen.     In  1886,  however,  two  German  investigators. 


132  LIVING  ORGANISMS  OF  THE  SOIL       [chap. 

Hellriegel  and  Wilfarth,  who  had  been  growing  different 
kinds  of  plants  in  pots  containing  pure  sand  supplied 
with   nutrient   salts   in   solution,  and   particularly   with 
varying    quantities   of  nitrogen,   noticed    that    certain 
plants    which    were    being    nitrogen-starved    suddenly 
recovered  their  vigour  and  began  to  grow  as  though 
they  were   freely  supplied   with   the   missing  element, 
nitrogen.     Such  plants  were  always  of  the  leguminous 
family — peas,  beans,  clovers,  lupins,  etc. ;  when  grown 
in  the  sand  with  little  or  no  combined  nitrogen,  they 
began  by  reaching  a  stage  when  they  had  used  up  all 
the  nitrogen  contained  in  the  seed,  after  which  they 
became  stunted   or   took   on   rather   a   light  yellowish 
colour.     Some     of    them,    however,    would     suddenly 
recover  from  this  debilitated  condition,  turn  green  again, 
and  begin  to  flourish  as  before.     When  such  plants  were 
shaken  out  of  the  sand  in  which  they  were  growing, 
their  roots  were  always  found  to  be  studded  with  small 
warty  nodules,  which  varied  in  size  from  that  of  a  pin's 
head  on  clover  to  something  as  large  as  a  walnut  on  the 
larger  perennial  leguminous  plants.     The  investigators 
at  last  made  certain  that  the  presence  of  these  nodules 
upon  the  roots  was  accompanied  by  an  increase  in  the 
amount  of  combined  nitrogen  in  the  plant,  an  increase 
that  could  only  have  been  obtained  by  the  fixation  of 
some  of  the  atmospheric  nitrogen.     The  nodules  were 
found  to  be  colonies  of  a  peculiar  form  of  bacterium, 
which  must  have  originally  come  from  the  soil,  because 
when  leguminous  plants  were  grown  on  sterile  sand  with 
suitable  precautions,  no  formation  of  nodules  nor  fixation 
of  nitrogen  took  place  until  there  had  been  infection  of 
the  plant's  roots  either  by  the  introduction  of  a  trace  of 
soil  or  of  an  infusion  of  a  nodule  that  had  grown  on 
another  plant.     Once,  however,  inoculation   had   been 
effected,  nodules  began  to  appear  on  the  root  and  fixation 


VII.]  THE  NODULE  ORGANISMS  133 

of  nitrogen  took  place  in  the  plant  as  a  whole.  The  seat 
of  this  fixation  was  then  demonstrated  to  lie  in  the 
bacteria  themselves — these  bacteria  in  fact  live  in 
partnership  (symbiosis)  with  the  clover  or  other  plants. 
They  receive  from  its  leaves  a  certain  amount  of  sugar 
which  they  oxidise  in  order  to  obtain  the  energy 
necessary  to  effect  the  combination  of  free  nitrogen  ;  in 
their  turn  they  give  up  to  the  plant  the  nitrogen  com- 
pounds it  requires  for  its  life  cycle.  It  has  since  been 
found  possible  to  cultivate  these  bacteria  apart  from  the 
leguminous  plant  with  which  they  are  usually  associated 
and  to  get  them  to  fix  nitrogen  when  fed  with  sugar, 
but  the  amount  so  fixed  is  small  compared  with  that 
obtainable  by  them  for  the  living  plant.  The  organism 
has  also  been  shown  to  exist  in  several  different  forms ; 
in  the  soil  it  takes  a  motile  form  very  much  smaller 
than  the  rod-shaped  and  even  branched  forms  which 
may  be  seen  in  the  nodules,  and  in  this  state  it  is 
capable  of  passing  through  the  fine  root  hairs  of  a 
seedling  leguminous  plant,  an  infection  which  soon 
becomes  evident  in  the  growth  of  a  nodule  swarming 
with  the  transition  forms  of  the  bacterium.  The 
laboratory  evidence  for  the  fixation  of  nitrogen  by  these 
bacteria  was  thus  rendered  very  complete ;  indeed  it  is  a 
simple  matter  to  arrange  an  experiment  to  demonstrate 
the  process.  All  that  is  necessary  is  a  series  of  small 
pots,  preferably  of  glazed  earthenware,  filled  with  sand 
mixed  with  i  per  cent,  of  calcium  carbonate,  the  pots 
and  their  contents  being  sterilised  before  starting  by 
heating  in  an  oven  for  an  hour  or  so.  The  seeds  of 
some  leguminous  plant — vetches,  sweet  peas,  or  lupins  are 
convenient  for  the  purpose — are  sterilised  by  washing  in 
alcohol  and  recently  heated  distilled  water,  and  are  then 
germinated  on  sterile  sand  or  filter  paper  until  the 
seedlings  are  large  enough  to  handle.     Meantime  the 


134  LIVING  ORGANISMS  OF  THE  SOIL        [chap. 

pots  of  sand  are  watered  nearly  up  to  saturation  with  a 
culture  solution  made  up  as  on  p.  45,  but  omitting  the 
sodium  nitrate,  and  the  seedling  plants  are  carefully 
introduced.  The  pots  must  now  be  either  kept  in  a  case 
well  protected  from  dust,  or  the  surface  of  the  sand 
must  be  covered  over  by  a  thin  layer  of  cotton-wool,  in 
order  to  keep  away  dust  which  may  contain  the  organism. 
After  a  week  or  so  of  growth  all  the  plants  will  begin  to 
show  the  want  of  nitrogen  by  a  cessation  of  growth  and 
the  sickly  yellow  colour  of  the  leaves.  To  one  pot 
is  then  added  a  few  cubic  centimetres  of  a  soil  extract 
made  by  shaking  up  soil  with  rather  more  than  an  equal 
bulk  of  water,  and  filtering ;  to  a  check  pot  is  added 
the  same  quantity  of  infusion  that  has  been  boiled. 
Similarly,  another  pot  is  watered  with  an  infusion  of  a 
nodule  taken  from  a  leguminous  plant  of  the  kind 
growing  in  the  pot,  and  as  before  the  check  pot  is  given 
the  same  infusion,  boiled.  The  checks  will  continue  to 
get  more  sickly  looking  and  will  shortly  die,  whereupon 
it  can  be  ascertained  that  they  possess  no  nodules  on 
their  roots ;  but  the  plants  inoculated  either  from  soil  or 
another  nodule  develop  nodules,  recover  their  health, 
and  will  grow  into  perfect  plants  if  the  supply  of  mineral 
constituents  be  sufficiently  renewed.  The  experiment 
should  be  duplicated,  because  the  check  pots  are  liable 
to  get  accidently  inoculated  unless  dust  is  most  rigor- 
ously excluded. 

This  discovery  of  the  fixation  of  nitrogen  by  the 
bacteria  associated  with  leguminous  plants  at  once  threw 
light  on  a  number  of  facts  which  had  been  difficult  of 
explanation  before.  For  example,  farmers  had  always 
known  that  in  some  way  the  growth  of  clover  and  similar 
plants  was  beneficial  to  the  soil ;  even  in  the  time  of  the 
Romans,  Pliny,  Virgil,  and  other  writers  on  agriculture 
had  instructed  the  husbandman  to  prepare  his  land  for 


vn.]  VALUE  OF  LEGUMINOUS  CROPS  135 

wheat  by  first  growing  crops  of  vetches  or  lupins  or 
beans.  In  English  agriculture  the  same  idea  was 
enshrined  in  the  Norfolk  four-course  rotation,  in  which 
red  clover  or  beans  are  followed  by  wheat ;  and  it  was 
well  known  to  the  practical  man  that  the  wheat  always 
flourished  best  where  the  clover  had  been  good  in  the 
previous  year,  Boussingault,  who  was  the  first  scientific 
man  to  make  field  experiments,  drew  up  a  kind  of 
balance-sheet  in  which  the  carbon  and  nitrogen  sup- 
plied to  the  land  over  a  term  of  years  was  com- 
pared with  the  amounts  of  the  same  elements 
taken  away  in  the  crops.  Of  course,  in  all  cases  the 
carbon  removed  was  enormously  in  excess  of  that 
supplied,  because  of  the  assimilation  that  had  taken 
place,  but  when  the  land  was  alternately  bare  fallowed 
and  cropped  with  wheat,  there  was  no  more  nitrogen 
obtained  in  the  crop  than  had  been  put  on  in  the  manure. 
But  with  other  rotations  including  clover  and  beans, 
there  was  always  more  nitrogen  harvested  than  was 
applied ;  and  when  the  land  was  occupied  by  lucerne  or 
alfalfa,  more  than  a  hundred  pounds  of  nitrogen  were 
taken  away  annually  for  five  years,  and  yet  the  soil 
showed  no  sign  of  impoverishment,  rather  the  contrary. 
Another  example  of  the  same  kind  was  obtained  at 
Rothamsted  :  in  1873  a  piece  of  land  was  divided,  one 
part  being  cropped  with  barley,  the  other  with  clover 
which  had  been  sown  the  year  before ;  the  nitrogen  was 
determined  in  the  two  crops,  and  showed  that  in  the 
barley  37  lb.  of  nitrogen  was  removed,  in  the  clover  151 
lb.  In  the  following  year  the  whole  of  the  land  was  sown 
with  barley,  and  the  crop  where  barley  followed  barley 
contained  39  lb.  of  nitrogen  per  acre,  whereas  that  which 
followed  clover  contained  69  lb.  per  acre.  An  analysis 
was  also  made  of  the  soil  in  1873,  after  the  first  barley 
and  clover  crops  had  been  removed,  and  the  soil  after 


136 


LIVING  ORGANISMS  OF  THE  SOIL       [chap. 


clover,  down   to   a   depth   of  9   inches,   was   found   to 
contain  3915  lb.  per  acre  of  nitrogen,  against  3540  lb. 
after  barley.     Table  XIII.  sets  out  the  figures. 
Table  XIII. — Effect  of  Growth  of  Clover  on  Succeeding  Crop. 


Nitrogen  in  Crop  and  Soil.    Lb.  per  Acre. 


Plot  A. 


Plot  B. 


In  Crop  (1873),  Barley  .        .      37-3 
In  Soil  after  Barley  .         .     3540 

In  Crop  (1874),  Barley  .         .      39.1 


Clover    . 
After  Clover 
Barley    . 


151-3 
3915 
69.4 


This  last  experiment  illustrates  a  fact  of  great 
practical  importance — that  a  good  crop  of  clover  not 
only  produces  a  lot  of  highly  nitrogenous  fodder,  in 
which  most  of  the  nitrogen  has  been  won  from  the  air, 
but  also  leaves  behind  in  the  roots  and  stubble  such  an 
additional  amount  of  nitrogen  as  will  be  of  considerable 
benefit  to  future  crops.  Another  example  from  Rotham- 
sted  will  illustrate  this  point :  on  the  Agdell  rotation 
field,  which  is  farmed  on  the  Norfolk  four-course  shift, 
one-half  of  the  field  carries  no  crop  during  the  year  pre- 
ceding the  wheat,  whereas  the  other  half  grows  clover  or 
beans.  Thus  an  opportunity  is  afforded  of  estimating 
the  effect  of  the  clover  or  beans  on  the  succeeding  crops 
of  the  rotation.  If  we  take  for  our  example  the  years 
following  a  specially  good  clover  crop,  and  compare  the 
two  plots  which  are  manured  once  in  the  rotation,  we 
obtain  the  figures  in  table. 

Table  XIV.— Total  Produce  per  Acre  after  Clover  or  Bare 

Fallow. 


Clover,  1894. 

Wheat,  1895. 

Swedes,  1896. 

Barley,  1897. 

Clover  Plot      . 
Bare  Fallow  Plot     . 

767  cwt. 

5209  lb. 
4547  „ 

389  cwt. 
380    „ 

4913  lb. 
3595  n 

VII.] 


EFFECT  OF  CLOVER  CROP 


137 


Thus  we  see  that  the  clover  crop  not  only  produced 
nearly  4  tons  of  hay,  which  was  removed  from  the  land, 
but  left  behind  residues  which  increased  the  next  wheat 
crop  from  4  to  5  quarters,  and  even  caused  a  large 
increase  in  the  barley  crop  which  came  two  years  later, 
in  spite  of  the  fact  that  a  considerable  amount  of  nitro- 
genous manure  had  been  applied  to  the  intermediate 
crop  of  swede  turnips.  It  is,  in  fact,  possible  by  growing 
clover  once  during  the  rotation,  to  maintain  the  soil  at  a 
moderate  level  of  fertility  without  bringing  in  any 
external  source  of  nitrogen  such  as  purchased  fertilisers 
or  feeding  stuffs,  notwithstanding  the  continual  removal 
of  nitrogen  from  the  farm  in  the  corn  and  the  meat  that 
are  sold  away.  The  following  example  from  the  same 
Agdell  field  will  illustrate  the  point : — 

On  one  of  the  plots,  which  receives  phosphoric  acid 
and  potash  but  no  nitrogenous  fertiliser,  clover  is 
grown  once  in  the  four  years'  rotation,  and  the  swede 
turnip  crop  is  chopped  up  and  ploughed  in,  thus  re- 
turning to  the  land  the  fertilising  materials  contained 
in  the  plant,  in  the  same  way  as  is  done  when  the  turnips 
are  fed  off  with  sheep.    The  following  table  (XV.)  shows 

Table  XV.— Conservation  of  Soil  Fertility  during 
Rotation  when  no  Nitrogen  is  Supplied.    (Rothamsted.) 


Nitrogen  in  Soil,  per  cent.     .     1867 
•     1874 
.     1883 
.     1909 

0-138 
0-132 
0.138 
0.150 

rWheat    .         .        . 
Average  yield,  J  Clover  Hay    . 
1852  to  1903     1  Swedes   . 

iBarley    . 

35.1  bushels 
47-7  cwt. 
9'3  tons 
34'5  bushels 

that  the  nitrogen  in   the  soil  is  not  experiencing  any 
measurable  loss,  while  the  crop  returns  indicate  that  a 


138  LIVING  ORGANISMS  OF  THE  SOIL       [chap. 

fair  level  of  fertility  is  being  maintained  through  the 
nitrogen  collected  by  the  growth  of  clover  once  during 
each  four-year  cycle. 

All  these  illustrations  serve  to  emphasise  the  point, 
that  by  the  growth  of  leguminous  plants  as  frequently 
as  possible  the  farmer  possesses  a  means  of  maintaining 
and  even  increasing  the  stock  of  nitrogen  in  his  land 
without  expense,  because  the  crop  of  valuable  fodder 
produced  in  itself  pays  for  the  year's  farming.  Of  the 
crops  commonly  grown,  lucerne  (or  alfalfa)  seems  to  be 
the  most  effective  in  fixing  nitrogen ;  it  is  particularly 
valuable  on  poor  chalky  and  sandy  soils  where  it  can  be 
left  down  for  several  years,  when  it  will  with  a  minimum 
of  expense  both  yield  a  paying  amount  of  fodder  and 
prepare  the  land  to  carry  a  short  succession  of  arable 
crops  with  no  extraneous  nitrogen  supply.  Lucerne  has 
proved  more  difficult  to  establish  on  the  poor  clays,  but 
even  there  its  benefits  are  very  marked.  Sainfoin  is 
probably  as  valuable  a  nitrogen  collector,  but  it  has  been 
less  rigorously  tested  ;  while  the  clovers,  particularly  red 
clover,  are  especially  effective  considering  the  short 
time  they  occupy  the  soil.  Red  clover  is  most  valuable 
to  the  soil  if  it  is  kept  grazed  after  the  first  cut ;  a  second 
cut  leaves  less  nitrogen  behind,  because  in  the  late  summer 
the  nodules  on  the  roots  become  considerably  depleted  of 
the  nitrogen  previously  gathered  from  the  atmosphere 
in  order  to  make  the  second  growth,  while  a  crop  of 
clover  seed  is  even  more  exhaustive  of  anything  that 
may  have  been  stored  in  the  root.  The  annual 
leguminous  plants — beans  and  vetches — gather  less 
nitrogen  from  the  atmosphere,  and  are  therefore  less 
valuable  as  preparations  for  succeeding  crops ;  indeed, 
on  the  rotation  field  at  Rothamsted,  a  bean  crop  has 
proved  no  better  preparation  for  wheat  than  a  summer 
of  bare  fallow.     At  Rothamsted,  however,  the  beans  are 


VII.]  SOIL  INOCULATION  139 

pulled  and  not  cut,  thus  depriving  the  soil  of  a  consider- 
able residue  of  root  and  stubble.  Of  course  it  should 
not  be  forgotten  that  leguminous  plants  can,  and  do  to 
a  large  extent,  in  nature  feed  upon  the  combined 
nitrogen  in  the  soil,  and  it  is  probable  that  a  plant  like 
a  bean  utilises  the  nitrogen  of  the  soil  until  it  is  driven 
by  starvation  to  make  the  nitrogen  of  the  nodules  its 
source  of  supply. 

In  view  of  the  enormous  importance  to  practical 
agriculture  of  the  nitrogen-collecting  power  of  the 
leguminous  nodule  bacteria,  a  number  of  experiments 
have  been  made  all  over  the  world  to  see  if  anything 
could  be  gained  by  introducing  an  increased  or  possibly 
a  more  vigorous  supply  of  these  organisms  into  the 
soil.  It  has  been  shown,  in  fact,  that  although  there  is 
only  one  species,  as  it  may  be  called,  of  bacterium 
associated  with  leguminous  plants,  yet  that  species 
possesses  a  certain  amount  of  racial  adaptation,  so  that 
clover  which  has  been  infected  from  a  clover  nodule 
grows  better  than  if  it  had  been  infected  from  a  bean 
nodule.  In  some  cases  this  specialisation  has  gone  so 
far  that  the  particular  leguminous  plant  can  only  be 
infected  from  another  plant  of  the  same  kind  or  by  soil 
in  which  it  has  been  grown  ;  it  responds  very  indif- 
ferently to  the  neutral  form  of  organism  that  is  present 
in  cultivated  soils  and  adapts  itself  to  most  of  the 
leguminous  crops  it  meets.  Moreover,  from  time  to 
time,  particularly  in  new  countries  or  where  heath  or 
bog  or  salted  alkali  land  is  being  reclaimed  for  the  first 
time,  soils  are  met  with  which  do  not  contain  any 
nodule  organisms,  so  that  when  leguminous  plants  are 
sown  they  neither  develop  nodules  nor  fix  nitrogen. 
These  considerations  have  led  to  the  artificial  inocula- 
tion of  the  soil  with  the  organism  appropriate  to  the 
desired  crop,  either  by  spreading  over  the  land  a  small 


I40  LIVING  ORGANISMS  OF  THE  SOIL        [CHAP. 

quantity,  half  a  ton  per  acre  or  so,  of  soil  taken  from  a 
field  in  which  the  crop  in  question  had  before  grown  suc- 
cessfully, or  by  sprinkling  on  the  seed  an  artificial  culture 
of  the  organism.  A  number  of  artificial  cultures  of  the 
organisms  appropriate  to  such  crops  as  clover,  lucerne, 
sainfoin,  beans,  peas,  etc.,  have  been  prepared  and  can 
now  be  purchased  commercially  in  an  active  state. 
The  practice  recommended  has  been  to  add  the  small 
pure  culture  that  is  purchased  to  a  fair  bulk  of  water 
containing  sugar  and  nutrient  salts,  or  to  diluted 
separated  milk,  and  to  keep  the  mixture  for  a  day  or 
two  in  order  to  allow  the  bacteria  to  increase,  then  the 
seeds  are  dipped  in  the  liquid  for  a  moment  immedi- 
ately before  sowing.  The  results  of  such  inoculation 
trials  have,  however,  been  disappointing  from  a  practical 
point  of  view  ;  in  most  cases  the  soil  is  already  so  well 
stocked  with  the  usual  nodule  bacteria  that  the  introduc- 
tion of  a  few  more  in  the  inoculating  liquid  does  not 
affect  the  growth  of  the  leguminous  plant.  In  the  much 
rarer  cases  where  the  soil  is  entirely  without  the 
organism,  its  artificial  introduction  may  prove  of 
enormous  benefit  and  enable  the  crop  to  grow  where  it 
would  otherwise  have  starved.  But  very  often  the 
absence  of  the  organism  is  only  a  sign  that  the  soil  is  in 
some  way  unfit  to  carry  it,  and  some  improvements 
must  be  effected  in  the  soil  before  either  organism  or 
leguminous  plant  can  be  established.  Even  when  the 
soil  is  not  actually  in  a  condition  to  prevent  the 
organism  growing,  the  introduction  of  a  particular 
bacterium,  such  as  that  associated  with  lucerne,  cannot 
be  done  all  at  once,  or  with  the  rapidity  and  certainty 
of  growth  that  is  seen  when  even  a  single  bacterium  is 
added  to  a  sterile  medium.  The  strongest  inoculation 
that  is  practical  only  introduces  a  minute  new  element 
into  an  active  flora  already  in  possession  of  the  soil; 


VII.]  BACTERIA-FIXING  NITROGEN  141 

to  make  good  their  footing  they  have  to  compete  with 
the  already  overcrowded  population  of  the  soil,  the 
numbers  of  which  are  only  maintained  at  a  certain  level 
by  the  limited  extent  of  the  food  supply.  It  has  thus 
been  found  necessary  to  make  several  inoculations  and 
renewed  sowings  before  lucerne  can  be  regularly 
established  in  soils  to  which  it  was  entirely  new,  though 
when  one  successful  crop  has  been  grown  there  is  never 
any  difficulty  later  in  establishing  a  second.  At  present, 
however,  the  practical  applications  of  pure  cultures  of  the 
nodule  bacteria  to  the  inoculation  of  soil  are  distinctly 
limited,  and  the  enormous  returns  that  have  been 
promised  for  them  can  only  be  realised  in  very 
special  cases. 

Since  the  discovery  of  the  nitrogen-fixing  organisms 
associated  with  the  nodules  of  leguminous  plants  several 
other  bacteria  have  come  to  light  which  can  fix  nitrogen 
without  any  such  partnership  with  a  higher  plant.  For 
example,  among  the  typical  organisms  bringing  about 
the  decay  of  organic  matter  in  such  places  as  the  mud  of 
ponds  and  marshes,  there  occurs  a  bacterium  possessing  a 
limited  power  of  fixing  nitrogen,  but  which  we  have  reason 
to  suppose  does  play  some  part  in  keeping  up  the  stock 
of  nitrogen  in  soils.  But  the  most  actively  fixing 
organism  that  has  been  isolated  is  a  comparatively  large 
bacterium,  called  Azotobacter,  which  has  been  identified 
in  virgin  soils  from  all  parts  of  the  world.  Certain  char- 
acteristic features  possessed  by  Azotobacter  Qmh\Q  it  to  be 
readily  detected,  and  the  following  experiments  may  be 
performed  to  illustrate  its  action.  A  non-nitrogenous 
culture  medium  must  be  prepared  as  on  p.  122,  omitting 
the  ammonium  sulphate,  and  portions  of  100  c.c.  are 
placed  in  flasks,  i  gramme  of  glucose  and  half  a  gramme 
of  carbonate  of  lime  are  added  to  each,  after  which  the 
flasks  are  plugged  and  sterilised  in  the  usual  way.     From 


142  LIVING  ORGANISMS  OF  THE  SOIL        [chap. 

one  or  two  of  the  flasks  the  carbonate  of  lime  is  omitted. 
If  such  a  non-nitrogenous  medium  is  inoculated  with  a 
mixture  of  organisms,  only  those  which  are  able  to  draw 
upon  the  atmospheric  nitrogen  can  live  and  multiply, 
because  all  the  usual  bacteria  depending  on  a  supply  of 
combined  nitrogen  are  starved  out  and  must  remain 
dormant  until  their  necessary  food  comes  along.  The 
series  of  prepared  flasks  are  then  inoculated  with  about 
a  gramme  of  soil  each,  and  put  away  in  a  warm  dark 
place  to  incubate  as  usual.  After  a  week  or  ten  days' 
time  the  flasks  are  examined  ;  wherever  the  Azotobacter 
was  present  in  the  soil  the  liquid  in  the  flask  will  be 
covered  with  a  dark  brown  skin,  held  up  on  the  surface 
by  a  number  of  bubbles  of  some  gas  which  is  evidently 
being  freely  developed.  The  flasks  containing  no 
carbonate  of  lime  will  often  show  no  brown  scum  nor 
formation  of  gas,  unless  the  soil  sample  added  is  itself 
rich  in  carbonate  of  lime.  An  analysis  of  the  contents 
of  the  flasks  will  show  that  nitrogen  has  been  gathered 
— often  as  much  as  8  milligrams  of  nitrogen  are  fixed 
for  each  gramme  of  sugar  in  the  original  solution.  The 
glucose  that  has  been  added  is  necessary,  not  merely  as 
food  for  the  Azotobacter^  but  as  material  which  can  be 
oxidised  or  burnt  up  to  supply  the  energy  required  to 
bring  the  nitrogen  into  combination.  Almost  any 
carbohydrate  will  serve  the  purpose,  but  glucose  and 
mannite  appear  to  be  most  readily  oxidised.  The 
Azotobacter  is  really  a  very  powerful  oxidising  agent ; 
the  gas  which  is  given  off"  during  the  first  growth  in  the 
culture  flasks  is  carbon  dioxide  produced  in  this  way. 
During,  or  by  means  of  the  rapid  process  of  oxidation, 
some  of  the  nitrogen  which  is  present  is  also  drawn  into 
combination.  Perhaps  an  even  more  satisfactory  method 
of  demonstrating  the  fixation — more  satisfactory  because 
it  follows  the  sequence  of  changes  taking  place  in  the 


VII.]      NITROGEN  FIXATION  IN  VIRGIN  SOILS        143 

soil  in  nature — is  to  put  some  of  the  flasks  back  in  the 
incubator  and  leave  them  there  for  one  or  two  months. 
The  other  groups  of  bacteria  introduced  in  the  soil 
sample  resume  their  activity  as  soon  as  they  can  obtain 
some  of  the  combined  nitrogen  that  has  been  introduced 
by  the  Azoiobacter,  and  a  portion  of  the  compounds 
that  have  been  so  elaborated  is  broken  down  and 
successively  oxidised  into  ammonia  and  nitrates.  On 
eventually  testing  the  contents  of  the  flask  no  glucose 
will  be  found,  but  the  presence  of  nitrates  can  be  shown 
by  diphenylamine  in  the  usual  way.  Finally,  if  the 
flasks  are  put  in  the  light,  green  algae  will  begin  to  make 
their  appearance,  the  spores  having  been  introduced 
with  the  soil,  and  thus  a  growth  of  green  vegetable 
matter  is  seen  which  has  derived  the  whole  of  the 
nitrogen  it  contains  from  the  atmosphere  through  the 
agency  of  the  Azotobacter. 

It  will  easily  be  seen  that  so  widely  distributed  and 
so  active  an  organism  as  Azotobacter  may  have  played 
a  considerable  part  in  maintaining  and  even  in  creating 
the  stock  of  combined  nitrogen  possessed  by  the 
world.  The  one  factor  necessary  for  fixation  is  that 
the  soil  shall  be  kept  supplied  with  the  purely  car- 
bonaceous material  formed  by  plants  from  the  atmos- 
phere by  the  assimilation  process.  In  the  Rotham- 
sted  wheat  field  it  has  been  shown  that  little  fixa- 
tion has  taken  place  in  the  soil  of  the  unmanured  plot 
— only  enough  to  replace  the  small  losses  by  drainage, 
weeds,  etc.,  the  amount  removed  in  the  crop  being 
balanced  exactly  by  the  reduction  in  the  stock  of  nitrogen 
in  the  soil  during  the  period  under  investigation.  The 
crop  being  wheat  after  wheat  every  year,  a  very  small 
residue  of  roots  and  stubble  are  left  as  material 
which  the  Azotobacter  can  oxidise.  When  more  carbon- 
aceous   residues    are    left    behind,   as   on    grass   land, 


144  LIVING  ORGANISMS  OF  THE  SOIL        [chap. 

particularly  where  the  natural  herbage  is  allowed  to  fall 
and  decay,  there  is  abundant  material  for  the  purposes 
of  the  organism,  and  fixation  in  consequence  can  become 
a  marked  feature  in  the  history  of  the  soil.     Two  small 
portions  of  land  at  Rothamsted  have  been  allowed  to 
run  down  from  arable  into  a  mass  of  rough  grass  and 
weeds,  nothing  having  been  done  to  the  land  for  more 
than  a  quarter  of  a  century.     The  coarse  derelict  herbage 
that  has  sprung  up — natural  prairie — falls  back  to  the 
soil  every  year,  and  an  analysis  of  these  soils  before  and 
after  the  growth  of  the  wild  vegetation  show  a  notable 
fixation  of  nitrogen,  over  90  lb.  per  acre  per  annum  for 
the  twenty-five  years  during  which  the  land  has  been 
left  to  itself.     This  experiment  also  helps  us  to  under- 
stand how  forest  soils  get  enriched  with  nitrogen ;  the 
autumnal  fall  of  leaf  adds  a  large  amount  of  carbonaceous 
matter  to  the  soil  and  thus  enables  the  Azotobacter  to 
go  to  work.     The  enormous  accumulations  of  nitrogen 
in  the  deep  black  soils  of  the  prairies,  the  steppes,  and 
similar  areas,  probably  also  owe  their  origin  to  Azoto- 
bacter ;    for   in   these  soils  the  requisite  conditions  are 
fulfilled — carbonate  of  lime  exists  in  the  soil,  and  there 
is  a  steady  addition   of  carbonaceous   matter   as   each 
year's  growth  dies  down  in  the  autumn. 

We  are  now  in  a  position  to  get  some  general  con- 
ception of  the  work  of  the  bacteria  in  the  soil ;  the 
actual  numbers  are  very  great,  as  many  as  forty  millions 
have  been  estimated  in  a  gramme  of  a  garden  soil 
fairly  rich  in  organic  matter,  and  the  number  of  species 
that  can  be  distinguished  is  considerable.  It  is,  how- 
ever, better  to  consider  them  by  groups  according  to 
their  function,  than  by  species,  and  the  main  features  of 
the  chief  groups  have  been  set  out  above.  All  these 
different  groups  are  at  work  together  in  the  soil,  but 
which  will  predominate  and  what  degree  of  activity  it 


VII.]  BACTERIAL  ACTIONS  IN  SOIL  145 

assumes  will  depend  very  greatly  on  external  conditions, 
such  as  the  cultivation  the  land  receives.  Perhaps  the 
most  important  are  the  organisms  concerned  with 
preparing  the  food  for  plants ;  in  the  first  place  the 
putrefactive  organisms  break  down  proteins  and  kindred 
nitrogenous  material  into  successively  simpler  com- 
pounds, which  are  then  taken  in  hand  by  another  group 
of  bacteria  and  converted  into  ammonia,  though  it  is 
impossible  to  draw  any  hard  and  fast  distinction 
between  these  two  sets  of  organisms.  The  work  of 
the  ammonia  -  makers  is  concluded  by  the  nitrifying 
organisms,  which  complete  the  final  stage  of  oxidation 
of  the  nitrogen  compounds  and  produce  the  nitrates 
forming  the  chief  source  of  nitrogen  for  our  crop  plants. 
The  nitrifying  organisms  are  on  the  whole  so  much 
more  active  than  the  ammonia-makers  that  there  is 
rarely  more  than  a  trace  of  ammonia  to  be  extracted 
from  the  soil ;  the  rate  at  which  nitrates  are  ready  for 
the  plant  is  mostly  determined  by  the  rate  at  which  the 
ammonia-makers  can  produce  ammonia  to  be  nitrified. 
The  work  of  all  this  group  of  organisms  is  promoted  by 
the  same  factors  as  forward  the  growth  of  other  living 
creatures,  i.e.^  a  proper  degree  of  warmth  and  moisture 
and  a  due  amount  of  soluble  food  (in  this  case  the 
mineral  salts  found  in  plant  ashes).  It  is  also 
stimulated  by  such  special  conditions  as  an  abundant 
supply  of  air,  a  sufficiency  of  carbonate  of  lime,  and 
cultivation  to  distribute  the  bacteria  in  the  soil.  On  the 
other  hand,  under  certain  conditions,  as  when  sulphate 
of  ammonia  or  nitrate  of  soda  is  applied  to  the  soil, 
many  organisms  will  seize  upon  these  soluble  nitrogen 
compounds  for  their  own  nutrition  and  multiply  in  such 
numbers  that  an  appreciable  fraction  of  the  nitrogen  in 
the  fertiliser  is  withdrawn  in  order  to  build  up  the 
bodies   of  the   bacteria,  which   becoming  thereby  con- 

K 


146  LIVING  ORGANISMS  OF  THE  SOIL        [chap. 

verted  into  proteins  or  substances  akin  to  them,  require 
to  go  through  the  routine  of  decay  and  oxidation  before 
they  can  reach  the  plant.  Next  come  the  group  of 
denitrifying  organisms  which  deal  wastefully  with  the 
nitrogenous  reserves  of  the  soil,  converting  them  into 
nitrogen  gas,  which  possesses  no  value.  These  become 
active  when  the  soil  is  waterlogged  and  the  supply  of 
oxygen  cut  off;  their  action  is  also  promoted  by  an 
abundance  of  organic  matter.  Then  we  have  groups  of 
organisms  working  in  a  contrary  sense  by  fixing  nitrogen 
gas  and  so  enriching  the  soil ;  some  of  these  organisms 
live  free  in  the  soil,  others  enter  into  symbiotic  partner- 
ship with  the  leguminous  and  a  few  other  kinds  of 
plants.  Similarly,  for  the  non-nitrogenous  organic 
matter  we  have  one  group  of  oxidising  or  decay 
bacteria,  which  burn  up  the  carbohydrates  to  carbon 
dioxide  and  water  in  the  presence  of  air,  and  another 
group  working  under  anaerobic  conditions,  which 
produce  carbon  dioxide  and  marsh-gas  or  hydrogen ; 
both  groups,  but  especially  the  latter,  giving  rise  to 
humus  as  an  intermediary  product.  The  tendency  of 
these  various  organisms,  which  are  in  many  cases 
working  in  opposite  directions,  is  to  arrive  at  some 
state  of  equilibrium  appropriate  to  each  given  soil  and 
to  the  treatment  it  receives;  for  example,  in  an 
exhausted  soil  such  as  prevails  on  the  unmanured  plot 
on  the  Rothamsted  wheat  field,  the  activity  of  the 
ammonia-making  organisms  has  been  reduced  to  the 
point  that  the  nitrogen  thus  rendered  available  for  the 
crop  is  practically  balanced  by  the  nitrogen  gained  by 
the  fixation  organisms.  On  the  other  hand,  on  the  plot 
receiving  farmyard  manure  every  year,  to  which  about 
four  times  as  much  nitrogen  is  being  applied  as  is  being 
removed  in  the  crop,  there  is  no  longer  any  increase  of 
nitrogen   in   the   soil ;  the   activity   of  the  denitrifying 


VII.]  GOOD  AND  BAD  SOIL  ORGANISMS  147 

group  has  been  so  increased  by  the  excess  of  organic 
matter  that  it  maintains  the  balance  by  getting  rid  of 
all  the  surplus  of  nitrogen. 

The  "  condition  "  of  the  soil  in  the  farmer's  sense  of 
the  word — its  capacity  to  keep  the  crop  steadily  and 
fully  nourished — is  in  the  main  determined  by  the 
activity  of  the  ammonia-makers,  and  this,  temperature 
and  moisture  conditions  being  equal,  depends  wholly 
upon  their  numbers.  The  numbers  are  in  the  first  place 
conditioned  by  the  food  supply  of  the  soil,  and  secondly 
by  another  wholly  different  class  of  living  creatures 
which  have  recently  been  ascertained  to  play  an 
important  part  in  the  soil.  These  are  various  protozoa 
— distinctly  animal  organisms  a  thousand  times  larger 
than  the  bacteria  upon  which  they  prey,  the  numbers  of 
which  they  keep  down  to  a  certain  limit.  It  is  possible 
on  a  small  scale  to  kill  off  these  protozoa  and  yet  leave 
the  bacteria  active,  in  which  case  the  productivity  of  the 
soil  is  raised,  even  to  the  extent  of  being  doubled,  with- 
out any  additions  of  manure  :  whether  these  small-scale 
processes  can  be  adapted  to  our  fields  yet  remains 
to  be  seen. 

Within  the  soil  there  are  also  numerous  microfungi 
and  moulds  of  all  kinds,  many  of  which  are  breakers- 
down  of  protein  and  carbohydrate  matter  like  some  of 
the  bacteria,  while  others  are  more  harmful,  because 
they  compete  with  the  higher  plants  for  food,  or  even 
give  rise  to  diseases  like  "  finger-and-toe "  or  "  club 
root"  in  turnips  and  cabbages.  In  a  general  way  it 
may  be  said  that  bacteria  predominate  in  soils  which 
are  kept  neutral  by  a  sufficiency  of  carbonate  of  lime, 
but  that  fungi  become  abundant  and  harmful  as  soon  as 
the  soil  is  allowed  to  get  acid,  their  activity  being 
promoted  by  the  use  of  acid  manures  like  superphos- 
phates and  sulphate  of  ammonia.     While  we  are  very 


148         LIVING  ORGANISMS  OF  THE  SOIL     [chap.  vii. 

far  from  being  able  to  control  the  bacteria  and  other 
organisms  of  the  soil,  we  are  beginning  to  realise  both 
the  fundamental  importance  of  the  part  they  play  in 
the  nutrition  of  the  crop  and  the  manner  in  which 
they  can  be  affected  by  processes  and  materials  which 
can  be  applied  to  the  soil ;  and  though  so  far  we  have 
only  succeeded  in  explaining  results  that  the  practical 
farmer  had  arrived  at  by  experience,  our  knowledge 
must  in  time  lead  to  deliberate  and  conscious  advances 
in  the  way  of  utilising  their  powers  to  better  effect. 


CHAPTER  VIII 

THE  CHEMICAL  COMPOSITION   OF  THE  SOIL 

Plant  Food  found  in  Normal  Soils.  Dormant  and  Available 
Plant  Food.  Rotations  and  Plant  Food  in  the  Soil.  Systems 
of  Farming — Wasteful  and  Conservative.  Requirements  of 
Different  Crops  for  Fertilisers.  Types  of  Soil — Characteristic 
Weeds  and  Crops. 

It  has  already  been  stated  that  while  the  greater  part 
of  the  soil  consists  of  sand  and  clay,  materials  that  are 
of  no  service  as  food  for  the  plants,  there  are  also 
present  much  smaller  quantities  of  the  elementary 
substances — nitrogen,  phosphoric  acid,  potash,  lime,  mag- 
nesia, etc. — elements  which  we  have  found  in  plants  and 
learnt  to  be  necessary  to  their  nutrition.  In  Table  XII. 
a  series  of  chemical  analyses  of  characteristic  English 
soils  are  set  out,  and  though  the  number  of  soils  repre- 
sented is  not  great,  the  examples  show  almost  as  wide 
variations  in  composition  as  may  be  expected  anywhere 
in  the  United  Kingdom.  The  first  things  that  may  be 
noticed  in  these  analyses  is  that  the  range  of  variation 
in  composition  is  not  great,  in  few  of  the  soils  does  the 
nitrogen  fall  below  o-i  per  cent,  in  very  few  arable  soils, 
on  the  other  hand,  does  it  rise  above  0-3  per  cent. ; 
phosphoric  acid  may  be  as  low  as  o-o6  per  cent  in 
exceptional  cases,  but  very  rarely  will  it  be  higher  than 
0-2  per  cent. ;  potash  perhaps  shows  the  greatest  varia- 
tions, ranging  between  o-i  per  cent,  to  i-o  per  cent. 
These  comparatively  small   variations  hardly  seem  to 

149 


ISO      CHEMICAL  COMPOSITION  OF  THE  SOIL     [chap. 

account  for  the  great  differences  in  fertility  which  we 
know  to  exist  between  one  soil  and  another,  differences 
which  are  reflected  in  a  rent  of  5s.  an  acre  in  one  case 
and  £^  in  another.  The  next  point  that  requires 
consideration  is  the  comparatively  large  quantities  of 
plant  food  present  in  even  the  poorest  soils,  a  quantity 
which  is  enormous  in  comparison  with  the  amount 
required  by  ordinary  crops.  For  example.  Table  VI. 
shows  that  few  crops  take  more  than  100  lb.  of  nitrogen 
per  acre  from  the  soil,  or  50  lb.  of  phosphoric  acid,  or 
1 50  lb.  of  potash  ;  only  heavy  root  crops  will  remove  as 
much  as  this,  crops  of  cereals  require  about  one-half 
As  the  layer  of  soil  9  inches  deep  over  an  acre  weighs 
about  two  and  a  half  million  pounds,  the  one-tenth  per 
cent,  of  nitrogen  which  is  to  be  found  in  all  but  the  very 
poorest  of  soils  would  still  represent  as  much  as  2500  lb. 
per  acre,  or  sufficient  for  twenty-five  full  crops  of  roots 
or  fifty  full  crops  of  corn. 

Yet  at  Rothamsted,  where  an  attempt  has  been  made 
to  grow  both  roots  and  corn  continuously  without 
manure,  in  a  very  few  years  the  crop  of  wheat  fell  to 
about  13  bushels  per  acre  of  corn  and  10  cwt.  of  straw, 
while  the  mangold  crop  averaged  no  more  than  4  tons 
per  acre.  Nor  does  growing  crops  in  a  rotation  instead 
of  continuously  help  matters  much  :  on  the  Agdell  field 
at  Rothamsted,  where  the  Norfolk  four-course  system  is 
followed,  the  average  production  of  the  unmanured  plot 
has  been  16  cwt.  swedes,  16  bushels  of  barley,  9  cwt.  of 
clover  hay,  and  26  bushels  of  wheat.  When  grown  in  a 
rotation,  the  cereal  crops  have  maintained  their  yield  at 
a  much  better  level  than  when  grown  continously  on  the 
same  land,  but  the  preceding  crops  of  clover  or  sv/edes 
are  so  poor  that  the  land  is  practically  lying  fallow  for 
the  year  before  the  corn.  Some  clue  to  the  explanation 
of  the  difficulty  is  afforded  by  the  facts  which  have  been 


VIII.]  DORMANT  AND  AVAILABLE  PLANT  FOOD    15 1 

considered  in  the  previous  chapter,  as  to  the  changes 
which  the  nitrogen  compounds  in  the  soil  have  to  go 
through  before  they  can  feed  the  crop.  Proteins  and 
similar  compounds  of  nitrogen  are  of  no  service  to  the 
plant  until  they  have  been  transformed  by  bacteria  into 
ammonia  and  nitrates  ;  hence  the  amount  of  nitrogen 
available  for  the  plant  in  the  soil  at  any  moment  may 
be  but  a  small  fraction  of  the  whole.  Just  in  the  same 
way,  phosphoric  acid  and  potash  must  be  brought  into 
solution  before  they  can  enter  the  plant,  and  the  rate 
at  which  the  compounds  of  these  constituents  in  the 
soil  will  become  dissolved  depends  greatly  on  their 
nature  and  physical  condition.  For  rough  practical 
purposes  we  may  divide  the  plant  food  in  the  soil  into  a 
dormant  and  an  available  portion,  though  it  would 
probably  be  nearer  the  truth  to  consider  the  whole  as 
potentially  available,  yet  differing  greatly  in  its  degree 
of  activity.  It  is  indeed  impossible  to  draw  any  absolute 
line  of  distinction  between  the  dormant  and  the  avail- 
able— only  an  absolute  division  can  be  made  among  the 
nitrogen  compounds,  of  which  nitrates  and  ammonia 
might  be  classed  as  available  and  the  rest  as  dormant, 
but  even  then  some  of  the  dormant  materials  might  be 
rendered  available  a  few  days  or  even  hours  later. 
Again,  among  the  compounds  of  phosphoric  acid  we 
might  regard  dicalcium  phosphate  in  the  soil  as  avail- 
able and  iron  phosphate  as  dormant,  but  really  this  only 
means  that  the  calcium  phosphate  would  yield  a  solution 
containing  perhaps  a  hundred  times  as  mixh  phosphoric 
acid  as  would  the  phosphate  of  iron.  This  shows  how 
vain  it  is  to  hope  to  separate  the  available  from  the 
dormant  plant  food  by  means  of  a  solvent  which  will 
extract  the  one  and  leave  the  other  undissolved ;  in  any 
solvent,  however  weak,  both  dissolve  up  to  a  point,  and 
the  differences  are  only  of  degree  and  not  of  kind. .   The 


152     CHJEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

point  to  realise  is,  that  plants  feed  on  the  solutions  in 
the  soil  water,  and  when  the  compounds  in  the  soil  can 
renew  this  solution  quickly  and  maintain  it  at  a  compara- 
tively high   concentration,  they  may   be   described   as 
"available"  plant  foods,  whereas  the  "dormant"  plant 
foods  give  rise  to  soil  solutions  of  low  concentration, 
which  are   not   speedily  repaired   when  the   plant  has 
extracted   the  constituent  in  question.     In  the  case  of 
nitrogen,  the  renewal  of  the  solution  depends  upon  the 
attack  of  bacteria  upon  the  nitrogen  compounds  in  the 
soil ;    as   regards    phosphoric   acid    and    potash,   more 
purely  physical  and  chemical  actions  regulate  the  rate 
of  solution.     The  formation   of  the  soil  solutions  has 
already   been    discussed,   and    it   was   stated   that  the 
carbon  dioxide  excreted  by  the  roots  of  the  plant  has  a 
considerable  effect  in  aiding  the  attack  of  the  soil  water 
upon  the  mineral  constituents  of  the  soil.     It  is  easy  to 
show  in  the  laboratory  that  a  solution  of  carbon  dioxide 
in  water  is   a   better   solvent   of  such   phosphates  and 
potash  compounds  as  occur  in  the  soil  than  pure  water 
is — almost  in  proportion  to  the  amount  of  carbon  dioxide 
dissolved — and  that  such  a  solution  must  exist  in  the 
soil  is  evident  from  the  composition  of  the  gases  in  the 
soil.     In  most  soils  the  actual  solid  particles  only  occupy 
about  60  per  cent,  of  the  total  space,  the  rest   being 
filled  up  by  water  and  air.     When  the  soil  is  saturated 
with  water  the  air  is  expelled,  but  complete  expulsion  is 
only  possible  when  the  soil  is  saturated  very  carefully 
from   below,  so   as   gradually  to   displace  all   bubbles. 
Under  ordinary  conditions  some  air  is  always  entangled 
even  when  the  soil  is  soaking,  and  as  soon  as  drainage 
gets  established  there  will  be  20  per  cent,  or  more  of 
air,  even  though  the  soil  is  apparently  very  wet.     The 
air  in  the  soil,  however,  does  not  long  retain  its  original 
composition;  by  the  respiration  of  the   roots  and  the 


VIII.]  CARBON  DIOXIDE  IN  SOIL  GASES  153 

oxidising  action  of  the  decaying  bacteria,  oxygen  is 
always  being  replaced  by  carbon  dioxide,  until,  as  the 
various  determinations  of  the  composition  of  the  air  ex- 
tracted from  an  alluvial  pasture  soil  indicate,  there  may 
be  10  per  cent,  of  carbon  dioxide  present,  despite  the 
exchanges  that  are  always  going  on  at  the  surface  where 
the  soil  becomes  open  to  the  air.  Now  the  carbon 
dioxide  will  dissolve  in  the  soil  water  in  proportion  to 
its  concentration  in  the  air  with  which  it  is  in  contact, 
hence  a  solution  with  a  fair  amount  of  power  to  attack 
minerals  is  formed  as  soon  as  the  rain-water  has  been 
lying  for  some  little  time  in  the  soil  and  has  taken  up 
its  proper  proportion  from  the  soil  air  with  which  it  is 
in  contact.  In  a  few  cases,  as  with  peaty  soil,  it  is 
possible  that  the  organic  matter  may  yield  certain  acids 
to  the  soil  water  which  give  it  an  extra  solvent  power, 
and  the  bleaching  of  stones  and  sand  in  peaty  soils  is 
sometimes  taken  as  a  proof  that  this  happens ;  but  it  is 
doubtful  whether  carbon  dioxide  would  not  equally 
effect  the  solution,  aided  perhaps  by  reducing  actions 
caused  by  the  organic  matter.  In  the  same  way  it  has 
already  been  stated  that  the  supposed  solvent  action  of 
roots  may  be  put  down  to  the  carbon  dioxide  they 
excrete,  rather  than  to  any  fixed  sap  acids  that  exude 
from  the  roots  and  then  attack  the  soil  particles  with 
which  they  are  in  contact.  That  roots  have  a  powerful 
solvent  effect,  may  be  learnt  from  various  direct  experi- 
ments ;  in  one  case  it  was  shown  that  plants  growing  in 
a  mixture  of  sand  and  ground  rock  phosphate  could  get 
a  good  supply  of  phosphoric  acid,  whereas  similar  plants 
starved  when  growing  in  the  sand  alone,  though  they 
were  supplied  with  water  that  had  previously  filtered 
through  a  second  plot  of  sand  and  phosphate.  In 
another  experiment  plants  were  grown  in  powdered 
granite,  and   when  this  was  washed  after  growth   was 


154     CHEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

over,  a  certain  proportion  of  it  was  found  to  have  been 
converted  into  clay.  The  solvent  action,  however,  in 
both  these  cases  can  be  put  down  to  the  carbon  dioxide 
excreted  by  the  roots. 

Many  attempts  have  been  made  to  devise  processes  of 
analysis  which  should  discriminate  between  the  available 
and  the  total  plant  food  present  in  the  soil :  it  has  already 
been  pointed  out  that  an  ordinary  straightforward 
analysis  which  determines  all  the  nitrogen  in  the  soil 
and  all  the  phosphoric  acid  and  potash  which  can  be 
dissolved  out  by  strong  hydrochloric  acid,  shows  enough 
food  material  for  fifty  and  even  one  hundred  ordinary 
crops.  Consequently,  when  such  an  excess  is  revealed 
it  is  difficult  to  interpret  the  analysis,  or  to  base  upon 
it  any  recommendations  to  the  farmer  regarding  the  kind 
of  manures  he  requires.  Very  dilute  acids  have,  there- 
fore, been  suggested — a  solution  of  citric  acid  that  shall 
be  comparable  to  the  acid  sap  of  the  root,  or  a  solution 
of  carbon  dioxide  that  shall  be  like  the  soil  water — with 
the  idea  that  all  of  the  phosphoric  and  potash  which 
these  liquids  would  dissolve  might  be  regarded  as 
available.  It  cannot  be  said  that  these  methods  of 
analysis  have  provided  infallible  information ;  it  is  not 
possible,  for  example,  to  say  that  if  a  soil  contains  less 
than  a  certain  limited  amount  of  phosphoric  acid 
determined  by  this  method  it  follows  that  a  phosphatic 
manuring  is  required.  The  limit  to  be  adopted  will 
vary  with  the  texture  of  the  soil,  the  water  usually 
present,  the  amount  of  carbonate  of  lime,  the  crop  under 
consideration,  and  other  independent  factors.  For 
example,  a  sandy  soil  may  contain  much  less  phosphoric 
acid  than  a  clay  soil  and  yet  feed  the  plant  equally  well, 
simply  because  the  open,  aerated  soil  induces  a  much 
greater  root  development.  Again,  a  crop  like  wheat, 
with  its  long  period  of  growth  and  deep  roots,  is  much 


VIII.]  ELEMENTS  FOUND  IN' SOIL  155 

less  dependent  upon  mineral  manures  than  are  the 
shallow-rooted  crops  like  turnips  and  barley.  Soil 
analysis,  even  by  the  most  refined  methods,  is  chiefly  of 
value  when  the  analysis  of  the  particular  soil  under 
consideration  can  be  compared  with  a  number  of 
analyses  of  similar  soils  on  the  same  type  of  land ;  so 
as  to  ascertain  its  relative  deficiencies  or  excesses,  and 
interpret  them  in  the  light  of  what  has  been  ascertained 
beforehand  about  this  type  of  soil  by  experience  or  by 
field  trials.  If,  for  example,  the  average  percentage  of 
phosphoric  acid  in  soils  of  a  given  type,  defined  perhaps 
by  a  particular  geological  formation,  is  0-15,  and  it  is 
also  known  that  phosphatic  manures  usually  meet  with 
a  fair  response  on  such  land,  then  if  the  soil  of  a 
particular  field  on  that  formation  only  shows  0-I2  per 
cent,  of  phosphoric  acid,  it  will  be  safe  to  predict  the 
necessity  for  phosphatic  manures  on  that  field,  though 
on  other  soils  0-12  of  phosphoric  acid  might  indicate 
such  richness  in  that  constituent  as  to  remove  the  need 
for  phosphatic  manures. 

One  other  point  is  brought  out  in  a  series  of  soil 
analyses :  besides  a  considerable  uniformity  in  the 
proportions  of  nitrogen,  phosphoric  acid,  and  potash,  all 
soils  contain  very  much  the  same  substances.  It  is 
extremely  rare  to  find  that  any  one  of  the  regular 
constituents  is  missing,  i.e.,  soda,  potash,  lime,  magnesia, 
iron,  manganese,  alumina,  chlorine,  silica,  phosphoric 
and  sulphuric  acids  ;  and  it  is  almost  as  rare  to  find  an 
appreciable  quantity  of  any  other  element,  though  one 
or  two  occur  occasionally  in  small  quantities.  Now  this 
fact  puts  an  end  to  the  very  common  idea  that  differ- 
ences in  character  or  quality  in  the  crops  grown  on  a 
given  soil  are  due  to  specific  deficiencies  in  the  soil 
which  may  be  made  up  artificially.  Of  course,  there 
are  cases  where  a  crop  shows  certain  special  features 


156      CHEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

brought  about  by  lack  of  phosphoric  acid  or  lack  of 
potash,  because  these  substances  affect  the  whole  course 
of  development  of  the  plant ;  yet  when  the  apples  grown 
on  a  particular  field  are  always  very  red  or  the  wheat  is 
very  strong,  it  is  useless  to  expect  to  find  some  substance 
in  the  soil  to  which  the  redness  or  the  strength  is  due. 
It  seems  so  straightforward  to  analyse  the  "strong" 
wheat  and  ascertain  the  substance  which  causes  the 
strength,  and  then  proceed  to  find  the  same  substance 
in  the  soil  giving  rise  to  strength.  Unfortunately, 
things  do  not  happen  in  this  simple  way  :  the  differences 
in  quality  are  usually  due  to  very  subtle  differences  in 
the  construction  of  the  plants'  constituents,  and  not  in 
the  materials — carbon,  hydrogen,  nitrogen,  oxygen,  etc. 
— out  of  which  the  structures  are  made.  In  nearly  all 
cases,  also,  the  structure  is  mainly  determined  not  by  the 
plant  food  or  the  manure  available,  but  by  the  climate,  the 
water  -  supply,  the  temperature,  and  other  physical 
conditions  of  the  soil.  Just  in  the  same  way  the  old 
theory  must  be  erroneous  which  explained  the  value  of 
a  rotation  of  crops  by  supposing  that  each  plant  in 
turn  extracted  one  particular  substance  from  the  soil, 
and  so  temporarily  exhausted  the  land  for  itself  but 
not  for  other  crops  requiring  a  different  nutrient. 
We  see  that  no  single  crop  can  exhaust  the  soil  of  any 
of  its  essential  constituents,  we  see  also  that  all  crops 
take  out  very  much  the  same  elements  and  in  similar 
quantities — therefore  the  value  of  a  rotation  depends 
on  a  quite  different  set  of  factors.  Putting  aside  the 
economic  arrangement  of  the  labour  which  a  rotation 
permits,  its  great  value  lies  in  the  facility  it  affords  for 
cleaning  the  land  without  any  special  labour.  When- 
ever one  crop  is  grown  continuously  on  the  same  land, 
as  is  done  on  the  Rothamsted  experimental  plots, 
certain  weeds  which  are  favoured  by  the  particular  crop 


VIII.]  VALUE  OF  ROTATIONS  157 

always  get  an  extremely  strong  hold  on  the  land  and 
become  very  expensive  to  keep  under.  Diseases  and 
insect  pests  special  to  the  crop  tend  to  accumulate 
until  they  may  unfit  the  land  to  carry  the  crop.  The 
tilth  of  the  land  often  suffers  when  the  same  crop  is 
grown  continuously  on  it;  for  example,  when  wheat 
follows  wheat  there  is  very  little  time  to  prepare  the 
land  between  the  crops;  again,  if  swede  turnips  are 
repeated  for  the  second  crop  the  land  would  miss  the 
autumn  ploughing,  which  on  many  lands  is  a  very 
necessary  element  in  the  preparation  of  a  seed  bed  for 
turnips.  And  though  soil  exhaustion  in  any  strict  sense 
is  impossible,  yet  when  any  particular  crop  is  repeated 
there  will  always  be  one  particular  layer  of  soil  in  which 
the  roots  of  the  plant  chiefly  reside,  and  this  layer  is 
somewhat  affected  either  by  the  removal  of  its  plant 
food,  or  possibly  by  the  addition  of  some  injurious 
substance  due  to  the  plant.  At  any  rate  it  is  the  deep- 
rooted  plants,  wheat  and  mangolds,  which  seem  to 
flourish  best  when  grown  repeatedly  on  the  same  soil ; 
whereas  shallow-rooted  plants  like  turnips,  barley,  and 
specially  clover,  soon  begin  to  fall  off  in  yield  if  grown 
continuously  on  the  same  land.  The  theory  that  plants 
excrete  some  substances  specifically  poisonous  to  them- 
selves and  that  a  rotation  is  necessary  to  give  it  time  to 
decay,  has  been  revived  of  late  years,  but  the  evidence 
in  its  favour  seems  inconclusive,  nor  is  it  necessary  to 
set  up  so  remote  a  hypothesis  to  explain  the  niain 
factors  in  the  virtues  of  a  rotation. 

Before  we  can  leave  the  question  of  rotations  and 
the  amount  of  plant  food  in  the  soil,  it  is  desirable  to 
arrive  at  some  idea  of  what  is  removed  from  the  land 
during  an  ordinary  course  of  cropping,  for  on  that  will, 
to  some  extent,  depend  the  nature  and  quantities  of  the 
materials   which  must  be  returned   as   manure   if  the 


158      CHEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

fertility  of  the  soil  is  to  be  maintained.  As  we  shall  see 
later,  we  cannot  wholly  gauge  the  requirements  of  a 
given  crop  by  first  ascertaining  what  it  takes  away  from 
the  soil,  but  we  can  sum  up  the  effect  of  a  given  rotation 
and  learn  whether  it  is  likely  to  leave  the  land  richer  or 
poorer.  We  may  consider  arable  land  first,  and  assume 
that  when  farmed  on  the  Norfolk  four-course  rotation 
it  is  of  such  fertility  as  to  yield,  on  the  average,  4^ 
quarters  of  wheat  and  5  quarters  of  barley,  2  tons  of 
clover  hay  and  20  tons  of  roots.  We  may  also  assume 
that  only  corn  and  meat  are  sold  off  the  farm,  the  straw 
being  trampled  down,  and  some  of  it  fed  with  the  roots 
in  order  to  make  the  dung  which  comes  back.  When 
farmyard  manure  is  made  in  the  yards  we  may  expect, 
for  reasons  which  will  be  set  out  later,  that  only  one-half 
of  the  nitrogen  contained  in  the  food  will  find  its  way 
back  to  the  land  in  the  dung,  though  when  roots  are  fed 
off  by  sheep  folded  on  the  land  about  nine-tenths 
will  be  returned.  About  three-quarters  of  the  phos- 
phoric acid  and  practically  the  whole  of  the  potash  may 
be  expected  to  come  back  in  the  dung.  On  this  basis 
we  should  get  figures  somewhat  as  shown  in  Table 
XVI. — the  annual  loss  to  the  land  per  acre  would 
amount  to  about  31  lb.  of  nitrogen,  9  lb.  of  phos- 
phoric acid,  and  5  lb.  of  potash. 

As  regards  phosphoric  acid  and  potash,  there  can  be 
no  possible  compensating  agents  at  work  other  than 
feeding  stuffs  or  fertilisers  brought  on  to  the  farm  from 
some  external  source  ;  it  will,  therefore,  be  necessary  to 
use  about  240  lb.  per  acre  of  superphosphate  (33  per 
cent,  acid  phosphate)  once  during  the  rotation,  say  for 
the  swedes,  in  order  to  maintain  the  fertility  of  the  land. 
The  draft  on  the  potash  is  not  so  great,  and  on  the 
stronger  soils  we  may  expect  that  the  weathering  of  soil 
particles  will  render  available  a  sufficient  proportion  of 


VIII.] 


MAINTENANCE  OF  FERTILITY 


159 


the  dormant  stock  of  potash  to  obviate  the  necessity 
of  any  specific  potash  manuring :  on  the  h'ght  soils 
200  lb.  of  kainit  per  acre  once  during  the  rotation  would 
repair  all  losses.  As  regards  the  nitrogen,  however,  the 
figure  we  have  obtained  possesses  very  little  value  as  a 
guide  to  practice,  because  it  ignores  the  whole  group  of 
gains  and  losses  of  nitrogen  which  are  due  to  bacteria  in 
the  soil.  For  example,  we  have  assumed  that  the 
nitrogen  contained  in  the  clover  crop  has  all  been 
derived   from   the   atmosphere,  so   that   this   crop  has 


Table  XVI. 


■Fertilising  Constituents  per  Acre  Removed 
DURING  Four-Course  Rotation. 


Nitrogen. 

rhosphoric 
Acid. 

Potash. 

V^heat,  36  bushels,  less  2\  for  seed     . 
Wheat  Straw,  \  ton  fed,  rest  in  dung  . 
Swede  Turnips,  20  tons  fed 
Barley,  40  bushels,  less  3  for  seed 
Barley  Straw  trampled  into  dung 
Clover,  2  tons  Hay,  fed       . 

Total  for  four  years    . 

Average  loss  per  acre  per  annum   . 

Lb. 
37 
0.7 
56 
32 

54* 

Lb. 

14-8 
o-S 
3-6 

I4-0 

3-8 

Lb. 
10.8 

CI 

0.4 
90 

0.4 

125.7 

35-7 

20-7 

31-4 

8.9 

5-2 

*  Nitrogen  drawn  from  atmosphere  and  not  added  in. 

entailed  no  loss  to  the  soil ;  but  we  have  very  sound 
evidence  that  the  soil  will  have  gained  nitrogen  after 
the  growth  and  removal  of  the  clover  crop.  Moreover, 
half  the  nitrogen  in  the  clover-hay  fodder  is  returned  to 
the  soil  in  the  dung,  whereby  the  farm  will  have  been 
still  further  enriched  by  the  growth  and  consumption  of 
clover.  On  the  other  hand,  the  soil  suffers  losses  of 
nitrogen  which  cannot  be  estimated — losses  both  by 
drainage  and  by  bacterial  action — and  these  losses  grow 
larger  the  higher  the  level  of  richness  at  which  the  land 


i6o     CHEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

is  maintained.  A  better  idea  of  the  requirements  of 
land  under  ordinary  farming  conditions  may  be  obtained 
from  a  study  of  certain  of  the  Rothamsted  plots,  for 
which  a  balance-sheet  showing  the  gains  and  losses  of 
nitrogen  over  a  period  of  years  can  be  drawn  up. 

Taking  first  the  unmanured  wheat  plot   as   an  ex- 
ample of  land  reduced  to  a  very  low  level  of  fertility,  we 
learn  that  during  the  years  1852  to  1902  ityielded  on  an 
average  a  crop  of  13-1  bushels  of  wheat,  which  contained 
in  both  grain  and  straw  18  lb.  of  nitrogen.     During  this 
period  the  percentage  of  nitrogen  in  the  soil  had  fallen,  and 
the  nitrogen  lost  by  the  soil  is  approximately  equivalent  to 
that  removed  in  the  crop.     At  this  low  level  of  fertility, 
then,  the   nitrogen   that  had   also  been   removed    by 
drainage   and   bacterial   action   (and   such   losses  must 
exist)    had    been    replaced    by    recuperative     actions, 
doubtless  due  to  Azotobacter  and  kindred  organisms  in 
the  soil.     But  for  the  plot  receiving  farmyard  manure 
every  year,  the  nitrogen  removed  in  the  crop  together 
with  what  has  been  stored  up  in  the  soil  only  accounts  for 
about  one-half  of  the  nitrogen  applied  as  manure.     The 
wasteful  actions  due  to  drainage  or  bacteria  must  have 
enormously   increased  and  have  removed  the  balance, 
any  recuperative  actions  being  swamped  in  such  great 
losses.     Turning   now  to  one  of  the  rotation  plots  on 
Agdell,  we  find  that  when  clover  or  beans  were  grown 
once  in  each  four  years,  and  where  also  the  root  crop  was 
returned  to  the  land  (see  Table  XV.),  the  nitrogen  in 
the  soil  remained  stationary  despite  its  removal  in  the 
crops  of  wheat,  barley,  and  clover,  or  beans.     In  this  case, 
then,  the  recuperative  actions  (which  include  the  growing 
of  a  leguminous  crop)  were  able  to  balance  the  wasteful 
processes  in  the  soil  and  the  removal  of  the  grain  and 
clover  crops,   without   any  additions  of  nitrogen  from 
outside  the  farm.     The  level  of  production  was  not,  how- 


VIII.]  THEORIES  OF  MANURING  i6l 

ever,  high,  being  only  35  bushels  of  wheat  and  34 
bushels  of  barley,  and  though  it  might  have  been 
increased  had  the  straw  and  clover  hay  been  made  into 
manure  and  returned  to  the  soil,  only  a  level  of 
production  below  the  average  can  be  maintained  if  the 
land  is  left  to  natural  recuperative  actions.  As  soon  as 
larger  crops  are  aimed  at  and  the  soil  is  brought  into 
higher  condition  to  produce  them,  the  wasteful  actions 
are  increased  in  a  higher  ratio,  as  may  be  seen  from  the 
example  of  the  wheat  plot  dunged  every  year.  Conse- 
quently, to  get  the  land  up  to  the  level  of  5  qrs.  of 
wheat,  not  only  the  extra  nitrogen  contained  in  5  qrs. 
above  4  qrs.  must  be  added,  but  a  great  deal  more  must 
be  brought  in,  in  order  to  make  up  for  the  increased 
rate  of  loss  taking  place  in  land  in  higher  condition. 

We  thus  find  it  impossible  to  construct  a  balance- 
sheet  for  the  land  that  sets  off  the  removal  of  plant  food 
in  the  crop  against  the  original  stock  in  the  soil  and  the 
additions  in  the  manures,  because  any  such  summary  is 
upset  by  the  unknown  gains  and  losses  suffered  by  the 
nitrogen  in  the  soil,  nitrogen  being  the  most  important 
element  of  plant  food.  As  we  began  by  dismissing  the 
idea  that  we  could  decide  upon  the  manuring  of  a  given 
field  by  finding  out  by  analysis  some  particular  substance 
lacking  in  the  soil  which  could  be  supplied  as  manure, 
so  now  we  must  dismiss  in  its  turn  the  theory  that  for 
the  proper  manure  we  have  only  to  provide  what  the 
crop  will  take  away  from  the  soil.  The  first  theory 
made  too  much  of  the  soil,  since  it  assumed  that 
different  soils  possess  radical  differences  which  do  not 
exist ;  the  second  theory  fails  because  it  takes  no  account 
of  the  soil  at  all,  and  neglects  the  enormous  reserves  of 
plant  food  therein  contained.  In  fact,  no  general  theory 
of  manuring  can  be  drawn  up  which  will  predict  the 
appropriate  treatment  for  every  plant  beforehand — we 

L 


i62     CHEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

must  ascertain  by  experiment  the  specific  requirements 
of  each  crop,  and  adjust  the  manure  to  them,  taking 
also  into  account  the  character  of  the  soil  and  the 
style  of  farming,  whether  high  or  low. 

According  to  their  habits  of  growth,  different  crops 
possess  very  different  powers  of  feeding  themselves 
upon  the  stock  of  plant  food  in  the  soil — the  specific 
requirements  of  each  is  generally  the  particular  element 
the  crop  finds  some  difficulty  in  obtaining  for  itself. 
For  instance,  wheat  is  sown  without  much  previous 
preparation  of  the  soil,  and  makes  the  greater  part  of  its 
growth  during  the  cooler  portion  of  the  year  when 
bacterial  activity  in  the  soil  is  low ;  as  a  consequence,  the 
stock  of  nitrogen  compounds  in  the  soil  is  being  but 
slowly  converted  into  ammonia  and  nitrates  available  to 
the  plant,  and  this  particular  crop  becomes  specially 
dependent  on  a  supply  of  some  active  form  of  nitrogen 
as  manure.  On  the  other  hand,  the  wheat  plant 
possesses  a  very  extensive  root  system  and  has  a  long 
period  of  growth,  so  that  it  searches  the  soil  pretty 
thoroughly  and  is  well  able  to  pick  up  the  mineral  plant 
foods — potash,  phosphoric  acid,  etc. — which  it  requires. 
Hence,  on  the  normally  fertile  soil,  wheat  requires  no 
mineral  manures,  but  will  respond  to  active  nitrogenous 
manure  should  the  land  not  be  in  very  high  condition. 
Barley  supplies  an  instructive  contrast — it  is  a  spring- 
sown  crop,  and  for  it  the  land  gets  a  more  thorough 
preparation  than  for  wheat.  Nitrification  and  other 
bacterial  processes  rendering  available  the  nitrogenous 
compounds  of  the  soil  are  active  in  the  recently  stirred 
land  as  it  is  warming  up  in  April,  May,  or  early  June, 
so  that  the  barley  crop  requires  little  manurial  nitrogen, 
though  it  takes  away  from  the  soil  as  much  of  this 
element  as  does  the  wheat  crop.  Being,  however, 
shallow-rooted   and   possessing   but   a   short  period   of 


VIII.]  SANDY  SOILS  163 

growth,  it  is  very  dependent  on  a  manuring  of 
phosphates,  potash  being  less  necessary,  because  it  is 
fairly  abundant  on  any  but  the  lightest  soils.  Swede 
turnips  afford  another  illustration :  they  are  sown  late, 
when  spring  is  well  advanced,  and  after  a  very  extensive 
working  of  the  soil ;  during  growth  there  is  also  a  good 
deal  of  intertillage,  which  promotes  bacterial  activity, 
with  the  result  that  the  crop  requires  very  little  nitrogen 
in  its  manure  though  it  takes  away  two  or  three  times  as 
much  as  the  wheat  crop  does.  But  with  its  shallow  root 
system  the  swede  crop  is  greatly  in  need  of  phosphates, 
and  also  of  potash — perhaps  more  freqnently  than  is 
suspected.  The  specific  requirements  of  crops  will  be 
dealt  with  more  particularly  in  a  later  chapter  ;  but  soils 
show  certain  real  differences,  partly  chemical  and  partly 
physical,  which  affect  their  manuring  and  management 
and  may  be  now  considered. 

Sandy  soils  are  made  up,  as  we  have  said  before,  of 
the  coarser  grades  of  particles,  and  contain  as  a  rule  but 
a  small  proportion  of  clay ;  it  follows  that  they  are  easily 
and  rapidly  permeable  by  water,  and  as  they  do  not 
retain  much  water  they  warm  up  quickly  in  the  spring 
and  are  early  soils.  Possessing  but  little  clay,  they  not 
only  dry  quickly,  but  they  can  be  worked  when  wet 
without  any  danger  of  destroying  their  texture ;  this, 
again,  helps  towards  early  cropping.  Their  coarseness 
of  grain,  however,  prevents  them  from  lifting  subsoil 
water  readily  to  the  surface,  so  that  crops  on  them 
suffer  from  even  short  periods  of  drought,  and  it  is 
necessary  to  keep  them  as  consolidated  as  possible  by 
folding  with  sheep,  rolling,  etc.,  in  order  to  help  the 
capillary  uplift  of  water.  Being  so  warm  and  well 
aerated,  all  bacterial  actions  go  on  rapidly  in  light  soils, 
and  humus  and  other  organic  materials  decay  quickly : 
such  soils,  therefore,  are  not  retentive  of  manure    but 


1 64      CHEMICAL  COMPOSITION  OF  THE  SOIL     [chap. 

must  be  dressed  frequently  and  heavily  if  they  are  to 
be  made  fertile.  From  the  chemical  point  of  view,  they 
often  show  certain  special  deficiencies ;  very  generally 
they  are  lacking  in  carbonate  of  lime,  with  the  result 
that  turnips  and  similar  cruciferous  crops  are  found 
to  suffer  from  finger-and-toe  or  club  root.  Lacking 
clay,  they  are  also  apt  to  be  deficient  in  potash,  and 
crops  like  mangolds,  potatoes,  clover,  and  tobacco, 
which  require  plenty  of  potash,  should  always  receive 
potash  manures  on  sandy  soils.  All  sandy  soils,  from 
their  warmth  and  friability,  are  apt  to  be  very  weedy ; 
the  characteristic  weeds  are  often  those  associated  with 
lack  of  lime  rather  than  with  the  dryness  of  the  soil. 
Good  examples  of  this  type  of  weed  are  the  Spurreys 
{Spergula  arvensis  and  Spergularia  rubra),  Sheep's  Sorrel 
(Rmnex  acetosella),  and  Corn  Marigold  {Chrysanthemum 
segetum),  all  common  on  light  sandy  soils,  but  indicating 
also  great  lack  of  lime  if  not  actual  acidity.  The 
Knawels  {Scleranthus  annuus  and  perennis).  Knot  Grass 
{Polygonum  aviculare),  and  some  of  the  wild  Poppies 
{Papaver  dubiuni)  and  the  small  Bindweed  {Convolvulus 
arvensis),  are  among  the  most  troublesome  weeds  of 
sandy  arable  land  ;  sometimes  also  a  form  of  Bent  Grass 
{Agrostis  alba)  is  very  abundant.  On  the  grass  land 
various  tufted  species,  Cock's-foot  {Dactylis  glomerata), 
the  Oat  Grasses  {Arrenathemuvt  avenescens,  Avena 
flexuosa),  and  the  Soft  Brome  {Bromus  mollis),  are 
characteristic.  Leguminous  plants,  except  Bird's-foot 
Trefoil  {Lotus  corniculatus),  and  more  occasionally  some 
of  the  Vetches  like  Vicia  cracca,  are  not  abundant; 
though  various  weeds — Hardhead  {Centaurea  cyanus)^ 
Buttercup  {Ranunculus  bulbosus),  Rattle  {Rhinanthus 
crista-galli)  and  Silver  Weed  {Potentilla  anserind)  are 
common.  On  the  wastes,  Gorse  ( Ulex  europceus  and  nanus) 
Broom  {Cutisus  scoparius),  Heather  and  the  Heaths,  the 


VIII.]  LOAMS  165 

Bracken  Fern  {Pteris  aquilina),  and  the  Foxglove 
{Digitalis  purpurea)  are  typical,  all  of  them  being 
associated  with  lack  of  lime  also ;  while  the  character- 
istic trees  are  the  Spanish  Chestnut,  the  Silver  Birch, 
the  Holly,  and  many  Conifers,  particularly  Pinus 
pinaster. 

The  sandy  soils  pass  by  insensible  stages  into  the 
loams — indeed  many  of  the  most  fertile  loams  are  really 
sands  in  which  the  finer  silt  fractions  predominate. 
Properly  a  loam  is  marked  by  an  equable  distribution 
of  the  various  grades  of  particles,  with  enough  clay  and 
fine  silt  to  render  it  retentive  of  moisture  and  manure, 
but  with  sufficient  sand  to  keep  it  open  and  well  drained. 
Chemically,  also,  the  loams  might  be  described  as  well 
found,  without  any  specific  deficiencies  or  excesses ; 
particularly  the  proportion  of  carbonate  of  lime,  though 
it  may  be  small,  is  sufficient  to  maintain  the  neutrality 
of  the  soil.  The  loams  might,  on  the  one  hand,  be  said 
to  have  no  specific  flora;  on  the  other  hand,  certain 
plants  are  only  found  on  good,  free-working  soil  in  fair 
condition,  so  that  they  may  be  taken  as  indicative  of 
fertile  loams.  Among  trees  the  Elm  is  characteristic,  as 
are  clean,  well-grown  Thorn  hedges,  and  an  abundance 
of  Nettles  {Urtica  dioica)  in  the  hedgerows  and  waste 
places.  On  the  arable  land,  Chickweed  {Steilaria  inedia)^ 
Groundsel  {Senecio  vulgaris),  Fat  Hen  {Chenopodium 
album)y  Annual  Nettle  {Urtica  urens),  and  Sow  Thistle 
{Sonchus  oleraceus)  are  weeds  indicative  of  good  land. 
Goose  Grass  {Galium  aperine),  the  Speedwells  (  Veronica ^ 
sp.),  Pimpernel  {Anagallis  arvensis),  Henbit  {La^nium 
amplexicaule),  and  various  Spurges  {Euphorbia  peplus) 
etc.,  are  also  common.  On  the  pastures.  Perennial  Rye 
will  generally  be  the  most  characteristic  grass,  and  its 
smooth  leaf  gives  a  characteristic  shine  to  some  of  the 
best  grazing  land ;  White  Clover  is  also  abundant,  and 


i66     CHEMICAL  COMPOSITION  OF  THE  SOIL    [chap. 

the  herbage  is  generally  very  varied  and  forms  a  thick 
close  sole.  Buttercups  are  very  common  on  some  of 
the  alluvial  pastures,  and  are  sometimes  taken  as  a 
sign  of  overmuch  cake  feeding ;  many  Oxeye  Daisies 
{Chrysanthemum  leucanthejnum)^  on  the  contrary,  indicate 
that  the  land  has  been  allowed  to  get  too  poor. 

Just  as  the  sands  pass  insensibly  into  the  loams,  the 
latter  grade  by  degrees  into  the  clays,  the  most  pro- 
nounced examples  of  which  are  the  soils  resting  upon 
some  of  the  formation  developed  in  the  East  Midlands 
and  in  the  east  and  south-east  of  England— the  Oxford 
and  Kimmeridge  Clays,  the  London  and  Weald  Clays, 
being  the  most  marked.  A  true  clay  soil  is  cold  and  late, 
very  retentive  of  moisture  yet  suffering  severely  from 
a  drought  of  any  duration,  not  only  because  of  the  cracks 
it  develops,  but  because  of  the  slowness  of  the  capillary 
movement  of  water  from  below  and  the  restricted  root 
development  of  all  crops  upon  clay,  especially  when  the 
land  has  not  been  drained.  Clays  are  difficult  to  work, 
the  draught  of  all  tools  being  heavy,  and  great  care 
must  be  taken  to  catch  them  at  the  right  moment  of 
working ;  especially  in  spring  it  is  ruinous  to  their  tilth 
to  put  horses  on  them  when  they  are  in  the  least  wet. 
It  is  most  important  to  plough  heavy  soils  in  the  autumn 
and  leave  them  rough  through  the  winter,  so  as  to  get 
the  beneficial  pulverising  and  flocculating  action  of 
repeated  frosts  and  thaws ;  on  the  heaviest  soils  an 
occasional  bare  summer  fallow  is  desirable  because  of 
the  value  of  the  weathering  to  the  tilth.  Being  cool  and 
moist,  clay  soils  are  retentive  of  manure  ;  long  straw  and 
other  bulky  manures  are  of  value  to  open  up  the  texture 
of  the  soil ;  it  is  often  indeed  of  service  to  burn,  or  rather 
char,  some  of  the  clay  and  incorporate  it  with  the  rest 
of  the  soil.  Clay  soils  are  very  often  deficient  in 
carbonate   of  lime,   so   that   dressings   of  lime   are   of 


VIII.]  CLA  Y  SOILS  167 

special  value  both  in  supplying  this  needed  constituent 
and  in  bringing  about  the  flocculation  of  the  finest 
particles,  thus  improving  the  texture  and  render- 
ing the  soil  drier  and  warmer.  Phosphoric  acid 
is  also  deficient  in  many  clays,  and  as  plants  become 
naturally  somewhat  shallow-rooted  on  such  soils  a 
good  supply  of  phosphate  manure  is  often  extremely 
effective ;  potash,  on  the  contrary,  is  usually  abundant  on 
clay  soils.  The  typical  crops  of  clay  soils  are  wheat, 
mangolds,  beans,  and  permanent  pasture,  though  the 
latter  may  be  very  bad  unless  it  is  properly  managed. 
Weeds  are  not  so  abundant  as  on  other  soils,  but  a  few 
are  specially  troublesome,  especially  Black  Bent  Grass 
{Alopecurus  agrestis)  and  Field  Mint  {^Mentha  arvensis)  ; 
the  Rest  Harrow  {Ononis  arvensis)  is  often  a  bad  weed 
on  the  poor  pastures ;  the  Wild  Carrot  {Daucus  carotd), 
the  Teazel  {Dipsacus  sylvestris),  and  the  Primrose 
{Primula  vulgaris)  are  characteristic.  The  Oak  is  the 
typical  tree  of  clay  land,  with  Ash  in  wetter  places,  and 
Hornbeam  in  the  underwood  and  hedges.  Clay  land 
pastures  are  often  characterised  by  a  very  shallow- 
rooting  vegetation,  which  may  even  become  largely 
stoloniferous  (creeping  rooting),  with  such  difficulty  do 
the  roots  penetrate  the  stiff,  unaerated  soil ;  a  form  of 
Bent  Grass  {Agrostis  alba)  often  constitutes  the  bulk  of 
herbage;  Sweet  Vernal  Grass  {^Anthoxanthemum  odor- 
atum)  and  Crested  Dog's-tail  {Cynosurus  cristatus)  are 
also  common. 

Calcareous  soils  may  be  either  light,  as  when  derived 
from  the  Upper  Chalk,  or  heavy  and  sticky,  when  they 
contain  much  admixture  of  clay  as  they  do  on  the  Chalk 
Marl  and  several  of  the  argillaceous  limestones  in  the 
Midlands ;  but  in  all  cases  they  possess  a  very  special 
and  characteristic  flora.  The  wild  plants  are  very  full 
of  flowers,  and   the  copses   and   hedgerows   contain    a 


1 68      CHEMICAL  COMPOSITION  OF  SOIL     [chap.  viii. 

number  of  flowering  shrubs  that  are  seen  on  few  other 
kinds  of  land — the  Traveller's  Joy  {Clematis  vztalba), 
the  Mealy  Guelder  Rose  ( Viburnum  lantana),  the 
White  Beam  Tree  {Pyrus  aria),  the  Dogwood  {Cornus 
sanguinea)  being  most  characteristic ;  while  of  the  trees, 
the  Beech,  the  Yew,  and  the  Cherry  are  particularly 
associated  with  chalk  and  limestone  soils.  Leguminous 
plants  are  abundant  both  in  the  pastures  and  the  waste 
places,  the  Horseshoe  Vetch  {Hippocrepis  comosd) 
being  a  very  sure  indicator  of  a  calcareous  soil,  while 
the  natural  home  of  Sainfoin  and  Lucerne  {Alfalfa)  is 
on  the  warm,  chalky  soils  of  the  south  and  east  of 
England.  The  lighter  chalky  soils  are  notoriously 
weedy.  Fumitory  {Fumaria  officinialis)^  Dove's-foot 
Cranesbill  {Geranium  molle)^  and  Field  Buttercup 
{Ranunculus  arvensis)  being  among  the  most  abundant. 

Peaty  and  waterlogged  soils  also  develop  a  special 
vegetation  which  need  hardly  be  considered  in  detail, 
since  the  farmer  merely  wants  to  recognise  certain  plants 
and  other  indications  that  show  the  need  for  drainage 
either  in  spots  or  over  the  whole  field.  Patches  of  rushes 
always  indicate  stagnant  water  near  the  surface,  as  also 
the  various  sedges  known  to  the  farmer  as  Carnation 
Grass  {Luzula,  sp.).  The  occurrence  of  tufts  of  Aira 
ccespitosa  also  indicates  wetness  ;  and  a  stagnant,  water- 
logged condition  of  the  soil  is  shown  by  the  presence  of 
brown  rusty  deposits  in  the  ditches,  accompanied  by  an 
iridescent  scum  on  the  surface  of  the  water.  On  digging 
into  such  land  a  layer  of  peat  is  generally  found  below 
the  surface  vegetation,  and  below  that  a  layer  of  rusty 
oxides  of  iron  upon  the  surface  of  the  true  soil.  The 
treatment  required  in  such  cases  is  drainage  and  liming. 


CHAPTER   IX 

FOODS 

Composition  of  Cattle  Foods.  Nature  of  Carbohydrates,  Fat, 
Proteins,  Fibre,  Ash.  Processes  of  Digestion  in  the  Animal 
Body.  Digestibility.  Character  of  various  Concentrated 
Foods,  Cereals,  Roots,  Straw,  and  Hay.  Valuation  of 
Feeding  Stuffs. 

We  have  already  discussed  the  constituents  of  plants  ; 
these  constituents  in  their  turn  make  up  the  foods,  and 
have  to  be  considered  from  a  fresh  point  of  view  in 
dealing  with  the  nutrition  of  animals.  As  a  rule,  the 
composition  of  any  given  cattle  food  is  expressed  as 
follows  : — Decorticated  cotton-seed  cake  contains — 


Moisture 

.     =       8-2  per  cent. 

Fat    . 

.     =     II-9        » 

Proteins  or  Albuminoids 

.     =     46-2       „ 

Carbohydrates  (by  difference) 

.       =       21-2 

Fibre 

•     =       5-5 

Ash  (containing  sand=i'5  per 

cent.)  =       7-0       „ 

The  fat  really  represents  the  material  which  is  soluble 
in  ether;  when  dealing  with  oilcakes  and  similar  ripe 
foods  it  will  consist  almost  wholly  of  pure  fat,  but  in  the 
case  of  green  fodders  various  other  materials,  e.g.  chloro- 
phyll, are  dissolved  by  the  ether  in  the  process  of 
analysis  and  counted  as  fat.  Crude  fat  would  be  a  more 
correct  title.     The  proteins  (albuminoids  of  the  older 

169 


I70  FOODS  [chap. 

books  and  in  trade  documents),  again,  should  be  called 
crude  proteins  or  some  equivalent  term,  because  the 
figure  expressing  them  is  only  obtained  by  multiplying 
by  6-25  the  total  nitrogen  contained  in  the  food,  thus 
including  as  proteins  various  non-protein  nitrogenous 
compounds  such  as  the  amides,  amino-acids,  and  other 
bodies  intermediate  between  nitrates  and  proteins.  All 
green  fodders  contain  a  considerable  proportion  of  their 
nitrogen  in  this  "amide"  or  non-protein  form,  and  in 
their  case  the  true  proteins  are  sometimes  determined 
separately.  Crude  fibre  is  again  a  purely  conventional 
term  for  whatever  remains  undissolved  when  the  food 
has  been  digested  for  some  time,  first  with  dilute  acid 
and  then  with  alkali :  it  represents  very  approximately  a 
part  of  the  food  which  can  only  be  very  imperfectly 
digested  by  the  animal,  and  is  therefore  of  much  smaller 
value  as  food.  The  ash  is  a  figure  of  value  to  the 
analyst,  while  the  "  sand  "  (that  portion  of  the  ash  which 
will  not  dissolve  in  weak  acids)  provides  an  index  of  how 
far  the  food  is  contaminated  with  dirt,  mud,  sand,  etc. 
Finally,  all  the  rest  of  the  food  is  reckoned  as  soluble 
carbohydrates ;  obviously  the  figure  expressing  their 
percentage  contains  the  accumulated  errors  of  the 
analysis,  and  is  of  value  only  for  comparison  with  other 
foods.  Indeed  in  the  United  Kingdom  the  seller  of 
feeding  stuffs  is  under  no  obligations  to  state  the  amount 
of  carbohydrates  in  the  food  he  is  selling,  though  he 
must  state  the  percentage  of  moisture,  nitrogen,  and 
fat.  In  all  foods  we  find  these  constituents — fats,  pro- 
teins, carbohydrates,  fibre — and  though  great  differences 
exist  between  the  various  materials  thus  classed 
together,  according  to  the  food  in  which  they  occur,  our 
knowledge  is  still  too  imperfect  to  make  it  worth  while 
discriminating  between  them  in  an  ordinary  analysis. 
We  must  now  consider  the  processes  of  digestion 


IX  ]  THE  PROCESS  OF  DIGESTION  171 

which  these  foods  undergo  in  the  body  in  order  to 
become  available  for  the  nutrition  of  the  animal.  They 
have  to  reach  the  blood,  this  being  the  circulating 
medium  which  conveys  them  to  every  part  of  the  body, 
and  in  consequence  they  must  either  be  soluble  or 
become  converted  into  such  a  state  of  solubility  as  will 
permit  them  to  pass  through  the  walls  of  the  stomach 
and  intestines.  These  organs  form  the  alimentary 
canal,  from  which  there  is  no  direct  opening  into  any 
other  part  of  the  body.  The  process  of  digestion  takes 
place  as  the  food  is  passing  along  this  alimentary  canal ; 
whatever  is  not  absorbed  by  its  walls  forms  the 
undigested  part  of  the  food,  which  is  excreted  as  the 
faeces.  The  solution  of  each  of  the  constituents  of  food 
is  effected  by  one  or  more  of  a  series  of  enzymes  which 
are  secreted  by  the  animal ;  these  enzymes  being  exactly 
comparable  in  their  action  and  nature  to  the  diastase, 
etc.,  which  have  previously  been  described  under  plants. 
Considering  first  the  fat,  its  digestion  is  not  accom- 
plished until  the  food  has  left  the  stomach ;  at  this  stage 
the  food  materials,  which  had  previously  been  rendered 
acid  by  the  gastric  juice,  are  brought  into  an  alkaline 
condition  by  mixture  with  the  bile  and  the  pancreatic 
juice.  Amongst  other  enzymes  the  pancreatic  juice, 
which  is  also  mixed  with  the  food  at  this  stage,  gives 
rise  to  a  lipase  or  fat-splitting  ferment,  which  resolves 
the  fats  into  their  component  fatty  acids,  and  glycerin. 
These  later  bodies  are  capable  of  passing  through  the 
walls  of  the  intestine,  and  by  means  of  the  blood  and  of 
the  lymphatic  system  they  are  distributed  about  the 
body.  It  is  found  that  fat  stored  up  by  an  animal,  or 
excreted  as  milk,  does  to  some  extent  partake  of  the 
nature  of  the  fats  contained  in  its  food,  showing  that 
the  fatty  acids  (the  basal  material  of  the  fats)  are  not 
entirely  broken  down  before  the  fat  is  reconstructed  in 


172  FOODS  [CHAP. 

the  animal.  On  the  other  hand,  there  is  plenty  of  evi- 
dence that  the  fat  of  the  food  never  passes  into  the  body 
or  into  the  milk  without  some  change.  Once  digested, 
all  fat  that  is  not  stored  is  used  by  the  animal  as  fuel  in 
order  to  maintain  its  heat  and  provide  it  with  the 
energy  by  which  it  is  enabled  to  work.  In  this  process 
the  fat  is  completely  burnt  up,  and  its  elements  leave 
the  body  again  in  the  form  of  carbon  dioxide  and  water, 
having  undergone  just  the  same  change  as  would  have 
taken  place  had  the  fat  been  set  on  fire.  The  fat  is 
truly  burnt,  and  has  been  proved  to  give  out  the  same 
amount  of  heat  inside  the  animal  as  if  it  had  been 
burnt  in  a  lamp. 

The  carbohydrates  contained  in  the  food  also 
become  soluble  as  a  result  of  enzyme  action ;  the  sugars 
are,  of  course,  soluble  to  begin  with,  but  starch  is  first 
attacked  by  a  diastase  which  occurs  in  the  secretion  of 
the  saliva.  In  consequence,  the  digestion  of  starch  is 
active  in  the  first  stomach  and  during  the  "  chewing  of 
the  cud  "  in  ruminant  animals.  This  salivary  digestion 
is,  however,  suspended  in  the  stomach  proper,  because  of 
the  acid  reaction  there  set  up  ;  but  the  starch  is  attacked 
again  in  the  small  intestine,  after  the  influx  of  the 
alkaline  bile  and  the  pancreatic  juice,  which  gives  rise 
to  a  second  diastatic  enzyme.  The  digestion  of  cellulose 
and  the  more  soluble  portions  of  the  fibre  is  less  easy  to 
follow.  All  seeds  themselves  contain  an  enzyme  capable 
of  attacking  cellulose,  and  this  would  become  active  in 
the  alimentary  canal,  especially  in  the  long  digestive 
tract  of  ruminants,  in  which  the  food  remains  for  four  or 
five  days.  There  is  little  evidence  that  animals  them- 
selves secrete  any  cytase  or  cellulose-dissolving  enzyme. 
The  intestinal  tract,  however,  contains  bacteria,  which 
multiply  greatly  in  the  food,  and  are  able  to  break 
down  the  cellulose  and  fibre  into  various  simpler  bodies, 


IX.]  THE  PROCESS  OF  DIGESTION  173 

such  as  sugars  and  fatty  acids  that  are  capable  of 
absorption  by  the  blood,  together  with  gases  like 
methane,  and  to  these  bacteria  is  ascribed  the  main  part 
of  the  work  of  digesting  cellulose  and  similar  carbo- 
hydrates. 

All  carbohydrates — sugars,  starches,  or  soluble  cellu- 
loses— acts  as  fuel  for  the  body :  having  reached  the 
blood,  they  are  passed  on  to  the  living  cells,  and  there 
burnt  to  carbon  dioxide  and  water  in  order  to  supply 
energy.  When  more  carbohydrates  are  supplied  than 
the  animal  requires,  they  can  be  built  up  into  fats  either 
for  storage  or  for  milk  production,  carbohydrates  them- 
selves being  never  stored  in  the  animal  body  except  in 
the  liver,  which  contains  one  special  body  of  this  nature 
— glycogen.  The  combustion  of  the  carbohydrates  is 
not,  however,  always  so  complete  as  that  of  the  fats ;  a 
certain  proportion,  especially  of  the  cellulose  and  fibre, 
is  excreted  in  a  unoxidised  form  as  methane  or  marsh- 
gas,  and  to  this  extent  the  value  of  the  carbohydrate  as 
fuel  has  been  reduced. 

The  digestion  of  the  proteins  is  a  more  complex 
process.  The  stomach  secretes  a  gastric  juice  which  is 
acid  and  also  contains  an  enzyme  called  pepsin,  by 
which  the  proteins  are  first  attacked.  Then  in  the 
small  intestine  the  partly  digested  food  is  mixed  with 
the  pancreatic  juice,  which  possesses  an  alkaline  reaction 
and  generates  a  second  enzyme  called  trypsin,  which 
also  attacks  the  proteins.  Both  of  these  enzymes  break 
down  the  proteins  into  peptones,  albumoses,  and'  succes- 
sively more  soluble  amino-acids,  which  are  able  to 
diffuse  through  the  walls  of  the  intestine.  There  the 
simple  nitrogen  compounds  are  once  more  built  up  into 
proteins,  and  in  that  form  are  passed  into  the  blood  and 
led  to  the  different  parts  of  the  body ;  though  another 
view  is  that  the  soluble  nitrogen  compounds  enter  the 


174  FOODS  [chap. 

blood,  and  are  only  re-formed  into  proteins  by  the  cells 
which  remove  them  from  the  blood.  The  greater  part 
of  the  nitrogen  compounds  thus  digested  is  oxidised 
into  carbonic  acid  and  water,  the  nitrogen  part  of  the 
compound  being  eliminated  from  the  blood  by  the 
kidneys,  chiefly  as  the  urea  which  is  excreted  in  the 
urine.  A  part,  however,  of  the  digested  protein — and 
this  is  really  the  indispensable  part  to  the  animal — 
is  used  up  for  repairing  nitrogenous  waste  in  the  tissues. 
In  growing  animals  some  of  the  proteins  of  the  blood 
are  stored  up  as  lean  meat ;  also,  when  there  is  an 
excess  of  protein  in  the  diet,  some  of  the  carbon  will  be 
stored  in  the  animal  as  fat,  the  nitrogen  being  again 
excreted  as  urea.  When  an  animal  is  not  putting  on 
weight  at  all,  the  whole  of  the  nitrogen  that  is  digested 
is  excreted  again  as  urea  and  kindred  bodies  in  the 
urine ;  the  amount  of  protein  required  to  repair  the 
daily  waste  is  comparatively  small,  and  the  excess  is 
simply  treated  by  the  animal  as  fuel. 

The  fibre  in  the  food  analysis  does  not  truly 
represent  the  indigestible  portions  of  the  food,  for 
all  natural  foods  except  pure  oil,  sugar,  and  starch  are 
only  imperfectly  digested  ;  some  part  of  the  food  resists 
the  intestinal  enzymes  and  bacterial  processes  and  is 
excreted  from  the  body  in  the  faeces.  As  regards  the 
nitrogenous  compounds  of  food,  it  is  important  to 
remember  that  whatever  is  digested  is  afterwards 
excreted  as  soluble  urea  in  the  urine,  while  the 
undigested  portions  remain  in  an  insoluble  state  in 
the  dung.  The  non-protein  nitrogenous  bodies  pos- 
sess a  much  smaller  food  value  than  the  proteins ; 
as  fuel  they  are  not  so  valuable,  while  it  is 
still  a  matter  of  debate  to  what  extent  they  can 
replace  proteins  in  repairing  tissue  waste.  Appar- 
ently they  can  at  least  save  waste  of  the  proteins,  and  in 


IX.]         MINERAL  CONSTITUENTS  OF  FOODS  175 

ruminant  animals  they  can  become  converted  into 
proteins  by  the  action  of  the  bacteria  in  the  intestinal 
tract.  Little  need  be  said  about  the  mineral  constitu- 
ents of  the  food — the  ash — though  it  is  essential  to  build 
up  the  bones  of  the  animal,  and  to  supply  the  salts 
circulating  in  the  blood.  As  with  the  other  constituents, 
some  of  the  contents  of  the  ash  are  digested  and  excreted 
in  the  urine,  some  are  excreted  unchanged  in  the  dung. 
Salt  is  one  of  the  most  important  food  constituents, 
since  from  it  has  to  be  made  the  hydrochloric  acid 
which  is  secreted  in  the  stomach,  and  is  required  for  the 
digesting  processes  there  proceeding.  Phosphate  of 
lime  is  also  absolutely  necessary;  from  it  is  built  up 
the  framework  of  the  bones  as  well  as  the  phosphorus 
compounds  which  occur  in  all  parts  of  the  body. 
Young  animals  soon  become  diseased  if  fed  upon  foods 
in  which  this  constituent  is  deficient.  Of  course,  besides 
these  main  constituents  which  run  the  machinery  of  the 
body — the  fat,  carbohydrates,  proteins — foods  contain  a 
variety  of  other  bodies  which  give  flavour,  and  affect 
the  disposition  of  the  animal  towards  its  food,  although, 
as  far  as  is  known,  they  do  not  actually  alter  its  digesti- 
bility. The  question  of  flavour  is  as  yet  beyond 
scientific  treatment,  its  influence  must  be  left  to  the 
observation  and  judgment  of  the  skilled  feeder  of 
cattle. 

When  the  value  of  different  foods  comes  to  be 
considered,  especially  with  the  object  of  compounding 
rations  for  feeding  stock,  it  is  not  sufficient  to  know  the 
percentage  of  fat,  proteins,  etc.,  which  the  food  contains, 
we  must  also  know  how  much  of  each  is  digestible. 
For  example,  pure  fat  is  completely  digestible  by  the 
animal  so  that  it  is  wholly  burnt  up  within  the  body,  yet 
the  same  fat  when  disseminated  through  the  mass  of  a 
hard  seed  may,  to  a  greater  or  less  degree,  escape  the 


176  FOODS  [CHAP. 

attack  of  the  digestive  ferments  in  its  passage  through 
the  alimentary  tract,  and  so  be  in  part  excreted 
unchanged.  Obviously  the  test  of  non-digestibility  is 
the  occurrence  of  the  material  in  the  faeces,  hence  if  we 
wish  to  determine  the  digestibility  of  a  given  food  we 
must  maintain  an  animal  on  a  diet  consisting  of  the  food 
in  question  over  a  particular  period.  During  this  period 
the  weight  of  the  food  given  and  the  faeces  excreted  is 
exactly  observed.  Both  food  and  faeces  are  analysed 
by  the  similar  methods ;  the  difference  between  the  fat 
that  has  been  fed  and  the  fat  contained  in  the  faeces 
gives  the  proportion  of  this  constituent  that  has  been 
digested,  similarly  for  the  protein  and  the  carbohydrates. 
When  concentrated  foods  which  cannot  be  fed  by  them- 
selves are  in  question,  e.g.  linseed  cake,  a  preliminary 
diet  of  hay  alone  is  given  and  the  faeces  analysed,  then 
for  a  certain  period  a  known  amount  of  linseed  cake  is 
added  to  the  hay  diet,  and  the  change  in  the  composition 
of  the  faeces  is  determined.  Certain  errors  are  inherent 
in  this  process ;  in  the  first  place,  it  is  impossible  to 
mark  off  exactly  when  the  faeces  corresponding  to  a 
given  diet  begin  and  cease  to  be  excreted.  Then  a 
certain  amount  of  waste  tissue  containing  nitrogen 
which  had  previously  been  stored  in  the  body  is  always 
excreted  into  the  intestine,  and  so  gets  reckoned  as 
undigested  protein.  Again,  the  bile  and  other  secretions 
provide  matter  which  is  soluble  in  ether  but  is  not  fat. 
Still  the  results  obtained  are  close  enough  to  the 
processes  going  on  in  digestion  to  be  of  real  value. 
Greater  discrepancies  are  introduced  by  the  fact  that  the 
digestive  power  of  one  animal  differs  considerably  from 
that  of  another,  and  besides  this  personal  idiosyncrasy 
one  kind  of  animal  possesses  greater  digestive  powers 
than  another,  especially  for  the  cellulose  and  fibre 
portions  of  the  food.     It  has  already  been  mentioned 


IX.]  DIGESTIBILITY  177 

how  much  more  capable  are  sheep  and  cattle,  with  their 
complex  stomachs,  ruminating  habits,  and  lengthy 
intestines,  to  deal  with  fibrous  foods  than  are  horses  or 
even  pigs,  in  the  intestine  of  which  animals  the  food 
remains  for  a  much  shorter  period.  Hence  the  digesti- 
bility of  many  foods  depends  upon  the  animal  to  which 
it  has  been  fed,  but  these  variations  occur  much  more  in 
dealing  with  such  bulky  foods  as  hay  or  straw  than 
with  the  concentrated  feeding  stuffs.  Of  course,  varia- 
tions occur  in  the  food  itself;  for  example,  hay  cut  before 
it  is  dead  ripe  is  the  most  digestible,  and  the  nitrogenous 
matter  of  hay  is  better  digested  when  the  food  is 
rich  in  this  constituent.  In  fact,  we  may  say 
generally  that  rich  foods  are  better  utilised  than  poor 
ones,  without  regard  to  the  fact  that  they  contain  initially 
a  higher  proportion  of  valuable  constituents.  Only  when 
an  excess  of  food  is  given  does  its  digestibility  decrease, 
a  starving  animal  cannot  get  more  than  the  normal 
amount  of  nutriment  out  of  a  given  food.  Again,  the 
work  that  the  animal  is  doing  has  little  or  no  effect 
upon  the  digestibility  of  the  food  given  to  it.  Cooking 
does  not  appear  to  increase  the  digestibility  of  any  of 
the  usual  cattle  foods ;  in  fact,  the  digestibility  of  the 
proteins  is  reduced.  Drying  the  food,  as  in  making  hay 
from  grass  or  in  curing  maize  forage,  does  not  diminish 
the  digestibility  if  the  process  is  properly  carried  out. 
Making  the  grass  into  silage,  so  far  from  increasing, 
actually  reduces  the  digestibility  of  such  proteins  as 
escape  reduction  to  ^-proteins  by  the  process;  it  is  a 
mistake  to  suppose  that  silage-making  will  convert 
worthless  grass  and  similar  waste  products  into  valuable 
food.  It  has  also  been  shown  that  while  the  addition 
of  fats  or  proteins  to  a  comparatively  poor  diet  of 
hay  and  straw  and  roots  causes  no  reduction  in  the 
digestibility  of  either  the  original  diet  or  the  additions, 

M 


178  FOODS  [chap. 

yet  if  carbohydrates  are  added  to  a  hay  or  straw  diet  the 
digestibility  of  the  proteins  in  the  original  diet  is  reduced 
when  the  added  carbohydrates  amount  to  more  than  lo 
per  cent,  of  the  fodder.  Thus,  roots  should  not  be  added 
to  a  hay  and  straw  diet  in  greater  proportion  than  15 
per  cent  (reckoning  both  as  dry  matter),  unless  some 
concentrated  protein  food  be  given  at  the  same  time  in 
order  to  maintain  the  ratio  between  nitrogenous  and 
non-nitrogenous  constituents  of  the  food  at  a  higher 
level  than  i  to  8. 

Table  XVII.  (page  179),  which  is  copied  from  a  table 
compiled  by  Dr  Crowther,  of  Leeds  University,  sets  out 
the  average  composition  of  a  number  of  the  foods  most 
commonly  in  use,  together  with  the  percentages  of  the 
same  constituents  that  are  in  a  digestible  condition  and 
one  or  two  other  factors  which  will  be  explained  later. 

The  most  concentrated  of  all  foods  are  the  meals  and 
cakes,  the  latter  being  the  residues  left  after  crushing 
various  oil-bearing  seeds  in  order  to  extract  as  much  oil 
as  will  come  out  by  pressure  alone.  The  composition  of 
such  cakes  will  vary  with  the  nature  of  the  seed  and  its 
origin,  but  the  amount  of  oil  in  the  cake  can  also  be 
greatly  modified  by  the  extent  of  pressure  put  on  and 
the  temperature  at  which  the  crushing  is  conducted. 
With  linseed  cake  in  particular  it  is  customary  not  to 
extract  the  oil  as  fully  as  would  be  possible,  whereby 
the  cake  is  enriched  and  at  the  same  time  rendered 
softer  and  easier  of  digestion.  In  some  cases  the  oil  is 
extracted  by  chemical  means,  but  the  seed  residue  is 
then  generally  used  for  manure ;  as  a  rule,  rape  seed 
is  treated  in  this  fashion  because  it  is  rarely  pure 
enough  to  be  used  afterwards  as  cattle  food,  being  often 
mixed  with  a  considerable  proportion  of  mustard  seed. 
In  the  United  Kingdom  few  cakes  are  used  beyond 
those   derived   from   linseed   and   cotton   seed,  and   of 


IX.] 


COMPOSITION  OF  FEEDING  STUFFS 


179 


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i8o  FOODS  [chap. 

recent  years  from  soya  beans ;  but  earth-nut  and  palm- 
nut  cakes  are  sometimes  obtainable  and  many  other 
kinds  of  seeds  are  crushed,  though  the  residues  are 
usually  worked  into  some  of  the  compound  proprietary 
mixtures  which  are  so  widely  sold  as  oilcakes. 
Farmers  are  recommended  to  buy  pure  cakes  made 
from  one  kind  of  seed  only.  As  a  rule,  such  cakes  are 
cheaper  intrinsically  even  though  their  price  is  higher, 
and  they  are  much  less  subject  to  adulteration.  Their 
greatest  value,  however,  lies  in  the  fact  that  the  farmer 
knows  exactly  what  he  is  using,  and  can  come  to  definite 
conclusions  for  the  guidance  of  his  future  practice. 

These  oilcakes  constitute  the  richest  and  most  con- 
centrated of  all  cattle  foods,  in  most  cases  also  their 
digestibility  is  very  high.  At  the  head  stands  decorti- 
cated cotton  cake,  since  cotton-seed  meal  still  containing 
its  oil  unextracted  is  rarely  seen  in  the  United  Kingdom, 
though  it  is  commonly  employed-  for  fattening  cattle  in 
the  United  States.  The  cake  is  made  from  cotton  seed 
from  which  the  husks  have  previously  been  removed, 
the  undecorticated  cake  being  made  from  the  whole 
seed  after  the  cotton  fibre  has  been  ginned  off. 
Decorticated  cotton  cake  is  extremely  rich  in  proteins 
and  oil,  and  contains  but  little  fibre ;  while  it  is  an 
excellent  food  for  bullocks  fattening  and  for  dairy  cows, 
it  must  not  be  given  to  the  latter  anywhere  near  the 
time  of  calving,  nor  should  it  be  fed  to  calves.  There  is 
evidence  that  it  contains  some  substance  which  is  at 
times  disturbing  or  even  poisonous  to  cattle,  so  that 
cotton  cake  must  always  be  used  with  discretion. 
Undecorticated  cotton  cake  is  the  favourite  adjunct  to 
the  food  of  cattle  fattening  upon  grass,  especially  in  the 
early  spring :  it  has  certain  astringent  properties  which 
correct  the  action  of  the  young  grass.  When  fed  to 
milch  cows  cotton  cake  tends  to  harden  and  raise  the 


IX.]  OILCAKES  i8i 

melting-point  of  the  butter  made  from  their  milk. 
In  buying  undecorticated  cotton  cake,  care  should  be 
taken  to  see  that  it  has  not  got  heated  or  become 
mouldy — the  smell  and  taste  give  fair  evidence  in  this 
direction ;  of  course  an  analysis  is  of  great  importance, 
to  see  that  it  contains  no  undue  proportion  of  husk. 
The  same  precautions  must  be  taken  over  undecorti- 
cated cake,  of  which  many  very  inferior  samples  are 
made  containing  an  excess  of  husk,  sometimes  ground 
to  a  powder  to  disguise  it ;  an  excess  of  cotton  fibre  also 
occurs,  and  is  undesirable. 

Linseed  cake  is  the  most  highly  esteemed  of  all  the 
feeding  stuffs ;  it  is  always  relished  by  stock  and  never 
disturbs  their  health  in  any  way,  though  if  fed  in  large 
quantities  to  milch  cows  it  is  apt  to  render  the  butter 
too  soft  and  oily.  Linseed  cake  is  particularly  valued 
by  graziers  at  the  end  of  the  fattening  process,  because 
nothing  else  will  confer  the  sleek,  shining  appearance 
and  kindly  feel  of  the  skin  which  comes  to  an  animal 
"finished"  on  linseed  cake.  Linseed  cake  should  be 
analysed  and  examined  for  purity;  it  should  show  no 
reaction  when  tested  for  starch,  which  would  only  be 
present  as  an  impurity  due  to  the  seeds  of  weeds,  for 
linseed  itself  contains  no  starch.  Very  hard  cakes  also 
are  undesirable,  because  they  are  low  in  oil.  There 
is  evidence,  however,  that  the  very  high  content 
in  oil  which  is  often  attained — 12  per  cent,  or  over — is 
rather  an  expensive  luxury,  for  just  as  good  results  can 
be  obtained  with  poorer  cakes  supplemented  by  a 
corresponding  amount  of  carbohydrate. 

Soya-bean  cake  is  so  recent  an  introduction  that 
little  can  yet  be  said  of  its  specific  actions  or  character, 
but  it  appears  to  be  an  extremely  valuable  food  for  all 
classes  of  stock.  Gluten  meal  and  gluten  feed  are 
residues  obtained  in  the  manufacture  of  various  maize 


i82  FOODS  [CHAP. 

products,  such  as  starch ;  they  are  in  consequence  foods 
specially  rich  in  proteins,  and  have  proved  valuable  in 
feeding  milch  cows  and  for  young  stock  growing  or  in 
the  early  stages  of  fattening. 

Of  the  cereals,  oats  are  the  richest,  containing  the 
highest  proportions  of  both  proteins  and  fat ;  they  are 
valuable  for  all  classes  of  stock,  and  always  wholesome. 
Wheat  requires  more  careful  feeding,  but  barley  again 
forms  a  very  safe  general  food,  containing  a  large  propor- 
tion of  carbohydrates.  Experience  has  shown  that  the 
deficiency  of  barley  in  protein  is  best  corrected  by  the  use 
of  cotton  cake.  Maize  is  chiefly  a  carbohydrate-contain- 
ing food,  and  however  valuable  for  work  and  for  fattening, 
as  regards  its  proteins  it  must  be  considered  as  rather- a 
low-grade  type  of  food.  Beans  and  peas  are  both  rich 
in  proteins ;  they  are  amongst  the  most  valuable  of  foods 
for  either  young  growing  stock,  horses,  pigs,  or  sheep  ; 
an  admixture  of  bean  meal  is  also  good  in  a  ration  for 
milch  cows. 

Oat  straw  is  generally  a  little  richer  than  either 
barley  or  wheat  straw,  but  the  variations  in  composition 
between  oat  straw  from  different  fields  or  different 
varieties  is  greater  than  the  differences  between  oat  and 
barley  or  wheat  straw.  Only  one  analysis  of  straw  has 
consequently  been  given  in  the  table.  The  composition 
of  hay  is  also  very  variable,  because  differences  due  to 
the  making  are  also  added  to  the  differences  induced  by 
the  nature  of  the  herbage,  the  soil,  or  the  season.  Of 
the  different  roots,  potatoes  contain  by  far  the  most  dry 
matter,  (25  per  cent),  then  mangolds  with  about  12  per 
cent,  swedes  with  a  little  less,  and  turnips  with  less  than 
10  per  cent  They  are  all  in  the  main  carbohydrate 
foods :  the  potato  contains  starch,  the  mangold  sugar, 
while  swedes  and  turnips  contain  various  pectic  bodies 
as  well  as  sugars.     A  large  proportion  of  their  nitrogen 


IX.] 


VALUATION  OF  FEEDING  STUFFS 


M 


is  present  in  the  form  of  non-protein  compounds,  as  it  is 
also  in  cabbage  and  all  other  green  foods. 

It  is  impossible  to  value  feeding  stuffs  with  the  same 
measure  of  precision  which  attaches  to  comparisons  of 
fertilisers,  but  certain  estimates  may  be  drawn  up  which 
are  useful  in  considering  the  purchase  of  foods  of 
the  same  class.  Fats  and  carbohydrates  may  clearly  be 
compared  together,  since  both  sets  of  bodies  are  used 
solely  as  fuel.  Experiments  which  will  be  discussed 
later  show  that  fats  ^wo.  rise  to  about  2-3  times  as  much 
heat  as  carbohydrates,  whether  compared  in  the  calo- 
rimeter or  in  the  body,  so  that  we  may  value  i  lb.  of  fat 
at  2-3  times  the  value  of  i  lb.  of  carbohydrate.  Proteins 
besides  their  value  as  fuel  are  also  necessary  to  repair 
waste ;  while  no  relation  depending  upon  principle  can 
be  fixed  between  them  and  the  carbohydrates,  for 
commercial  purposes  they  may  be  taken  as  equal  to 
fats,  Le.  at  2-3  times  the  value  of  carbohydrates.  Thus 
if  we  wish  to  compare  the  values  of  decorticated  cotton 
cake  and  of  gluten  meal,  we  can  proceed  as  follows : — 


Constituent. 

Decorticated  Cotton 
Cake. 

Gluten  Meal. 

Fat       .         .         . 

Protein 

Carbohydrate 

Percentage. 

9x2.3 

41x2.3 

26 

Food  Units. 
20.7 

260 

Percentage. 

4x2.3 

38x2.3 

45 

Food  Units. 

9-2 

87.4 

4S-0 

141-0 

141-6 

It  would  be  far  more  correct  to  consider  the  di- 
gestible constituents  only  in  such  a  valuation,  in 
which  case  the  feeding  stuffs  set  out  above  would 
show  118  and  126  units  respectively,  figures  which 
ought  not  to  be  too  widely  departed  from  in  the 
relative   prices   given   for  the   foods.     In   the    market, 


1^4  FOODS  [chap.  IX. 

however,  it  will  be  found  that  the  prices  depend  upon 
very  different  conditions,  mainly  of  a  commercial 
character,  hence  the  grazier  who  values  out  the  feeding 
stuffs  on  offer  before  purchase  can  generally  obtain  a 
substantial  advantage.  In  a  subsequent  chapter,  how- 
ever, it  will  be  shown  that  such  a  basis  of  comparison  is 
only  of  value  when  dealing  with  concentrated  foods ;  a 
more  accurate  basis  of  judgment  is  afforded  by  the 
starch  equivalents  which  will  then  be  explained. 


CHAPTER   X 

THE  UTILISATION   OF  FOOD  BY  THE  ANIMAL 

Food  as  a  Source  of  Energy.  Heat  Value  of  various  Foods. 
Energy  consumed  in  Digestion  and  Internal  Work. 
Maintenance  Rations.  Feeding  for  Rapid  Work  or  Increase 
of  Weight.  Amount  of  Food  required  for  a  given  Amount 
of  Work.  Nitrogenous  Materials  required  to  repair  Tissue 
Waste.    Minimum  of  Protein  necessary. 

So  far,  we  have  only  discussed  the  composition  of 
various  foods  and  the  process  of  digestion  by  which 
they  reach  the  body  ;  it  is  now  necessary  to  consider  the 
purposes  which  these  foods  serve,  and  their  utilisation 
within  the  body.  We  will  begin  by  looking  at  foods 
from  the  standpoint  of  energy :  we  have  already 
indicated  that  an  animal  is  an  organisation  which  has 
to  be  continually  fed  with  some  external  supply  of 
energy  in  order  to  maintain  its  warmth  and  its  power 
of  doing  work.  It  resembles  water  running  down- 
hill, in  that  when  once  started  it  runs  on  of  itself;  the 
transformation  of  energy  continues  as  long  as  the 
material  containing  the  energy  is  supplied,  and  during 
the  transformation  there  is  a  certain  wastage  through 
change  into  low-grade,  unusable  forms  of  energy.  The 
food  and  the  oxygen  together  contain  stored-up  energy  ; 
like  a  reservoir  of  water  at  the  top  of  a  hill,  they  will  run 
down  into  carbon  dioxide  and  water,  the  energy  being 
liberated  as  heat,  just  as  the  water  will  have  parted  with 

185 


1 86    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

its  energy  as  heat  by  the  time  it  is  at  rest  again  at  the 
foot  of  the  hill.  During  the  running-down  process,  also, 
in  both  cases  a  certain  proportion  of  the  stored-up 
energy  can  be  converted  into  work  or  other  forms  of 
valuable  energy;  moreover,  both  processes  will  run  of 
themselves  if  the  sluice-gate  be  opened  in  one  case,  or 
the  mixture  be  ignited  in  the  other.  The  energy  of  all 
bodies,  i.e.  their  capacity  for  doing  work,  is  best 
measured  in  terms  of  heat,  the  unit  usually  employed 
in  this  sort  of  work  being  the  large  calorie,  which  is  the 
amount  of  heat  required  to  raise  the  temperature  of 
I  kilogram  of  water  by  i  degree.  This  calorie  has 
been  valued  against  other  kinds  of  energy ;  for  instance, 
if  the  heat  contained  in  the  calorie  were  transformed 
into  work,  it  would  lift  425  kilograms  i  metre  (or  1-4 
tons  I  foot) ;  similarly,  electricity  or  light  can  also  be 
measured  in  terms  of  calories.  To  take  an  example,  we 
can  say  that  a  given  weight  of  coal  possesses  a  certain 
amount  of  energy,  which  would  be  developed  and 
measured  as  heat  if  the  coal  were  burnt  and  the  heat 
communicated  to  a  known  weight  of  water.  But  if  the 
coal  is  burnt  in  a  steam  boiler,  and  the  steam  made  to 
drive  an  engine,  less  than  the  whole  of  the  energy  of 
the  coal  will  be  found  in  the  waste  heat  from  the  boiler 
and  steam,  because  a  certain  proportion  has  been 
transformed  into  work.  A  dynamo  will  transform  this 
moving  energy  into  electricity  with  but  a  small  loss  by 
friction  {j.e.  with  but  a  small  reconversion  into  heat), 
and  the  electricity  may  be  changed  again  into  work, 
light,  chemical  energy,  or  other  forms — the  ultimate 
result  being  a  running  down  into  low-grade  heat.  The 
original  stock  of  energy  is  never  lost  or  destroyed,  the 
sum  total  of  the  different  forms  developed  being  always 
that  originally  present  in  the  coal ;  even  when  finally 
degraded  into  heat,  there  are  still  the  original  number 


X.]  ENERGY  OF  FOOD  187 

of  calories  present.  The  difference  is  that  such  low- 
grade  heat  has  lost  its  effectiveness,  and  can  no  longer 
be  transformed  into  work,  light,  or  electricity.  As 
another  illustration  we  may  take  a  watch :  in  the  act 
of  winding  a  certain  amount  of  energy  is  communicated 
to  it,  and  remains  stored  in  the  energy  of  the  coiled-up 
spring.  Gradually  this  energy  is  transformed  into  the 
energy  of  motion  of  the  parts  of  the  watch,  which  is  just 
as  continuously  rubbed  down  by  friction  into  heat  which 
leaks  away.  There  is  no  loss  of  energy;  the  world 
when  the  watch  runs  down  contains  a  little  more  evenly 
distributed  heat,  exactly  balancing  the  energy  com- 
municated to  the  coiled  spring  at  the  outset. 

The  total  energy  possessed  by  any  food — that  which 
is  often  called  its  fuel  value — can  be  measured  in  the 
simplest  fashion  by  actually  burning  in  oxygen  a  given 
weight  of  the  food  in  a  vessel  surrounded  by  a  known 
quantity  of  water  and  observing  the  rise  of  temperature 
that  ensues.  This  gives  the  maximum  amount  of 
energy  available  from  the  food,  but  the  animal  does 
not  realise  the  whole  of  it  except  in  the  case  of  a  pure 
fat  or  oil  which  is  as  completely  burnt  in  the  body 
as  it  is  in  the  calorimeter.  Most  foods  are  not 
completely  digested,  hence  the  excreta  contain  some 
energy  and  must  also  be  burnt  separately  in  the  calo- 
rimeter, the  heat  evolved  being  deducted  from  the 
total  energy  of  the  food  in  order  to  estimate  what  is 
available  for  the  animal.  The  nitrogen  of  proteins, 
again,  is  excreted  as  urea,  which  is  combustible  and 
contains  energy ;  this  also  must  be  burnt  and  the  heat 
deducted  from  the  fuel  value  of  the  food  ;  also  from  all 
carbohydrates,  and  particularly  from  fibre  a  certain 
proportion  of  methane  and  hydrogen  is  produced  in 
the  intestine,  and  these  unburnt  gases  represent  losses 
of  energy  to   the   animal.     For   example,  with   oxen, 


1 88    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

Kellner  obtained  the  following  fuel  values  and  heat 
values  for  certain  typical  foods,  expressed  in  calories 
per  gramme  of  food  : — 

Table  XVIII.— Energy  Developed  by  Various  Foods. 


Fuel 

Value 

per  grm. 

Losses  of  Energy. 

Heat 

Value  to 

Ox. 

Excreta. 

Urine. 

Marsh. 
Gas. 

Earth-nut  Oil  (Fat) 
Wheat  Gluten  (Protein) . 
Starch  (Carbohydrate)    • 
Meadow  Hay 
Wheat  Straw  . 

8.8 
5-8 
4-1 

3-6 

1-75 

2-1 

... 

I'l 

0-2I 
O'l 

0.4 

0-23 

0-3 

8-8 
4-7 
3-7 
1-8 
i-i 

This  table  shows  us  that  though  in  the  calorimeter 

fat  will  develop  —  =2-15  times  as  much  heat  as  starch, 

8-8 
yet  in  the  ox  it  will  develop  — 1  =  2-4  times  as  much, 

because  in  the  digestion  of  the  starch  some  of  the 
material  is  excreted  as  methane  which  is  still  com- 
bustible and  contains  energy. 

in  the  calorimeter  have  a  heat  value 

4.1 


Similarly  proteins,  which 

5-8 

—  1-4  times 


that  of  starch  in  the  animal,  possess  —  =  1-25  times 

the  value  of  starch,  because  of  the  comparatively  large 
proportion  of  undeveloped  energy  still  contained  in  the 
excreted  urea.  We  can  then  take  as  a  basis  these 
experimentally  ascertained  facts,  that  the  heat  value 
to  the  animal  of  a  pure  digested  fat  is  2-4  times  that 
of  starch,  and  that  of  digested  protein  is  1-25  times  that 
of  starch,  and  proceed  to   calculate  the  heat  value   of 


X.] 


HEAT  VALUE  OF  FEEDING  STUFFS 


189 


mixed  feeding  stuff  in  terms  of  starch.     For  example, 
ICK)  lb.  of  meadow  hay  (Table  XVII.)  contains : — 


Constituent. 

Percentage 
Digestible. 

Heat  Value  to 

Animal 
compared  with 

Starch. 

Total  SUrch 
Units. 

Fat 

Protein       .... 
Carbohydrates  and  Fibre  . 

I        X 

4     X 
41       X 

2.4 
1.25 

2.4 

5.0 

41.0 

48.4 

Thus,  100  lb.  of  meadow  hay  are  equivalent  to  48-4  lb. 
of  starch  by  calculation  from  the  constituents,  whereas 
Kellner  found  by  experiment,  as  shown  above,  that  when 
I  of  starch  gave  37  calories,  i  of  meadow  hay  gave 
1-8  calories,  or  100  of  meadow  hay  give  as  much  heat  as 

- — ^^-^  =  487  of  starch,  a  sufficiently  close  agreement. 

If  the  animal  is  at  rest,  either  fattening  or  on  a  mere 
maintenance  diet  {i.e.  the  minimum  diet  which  will 
keep  it  stationary  in  weight),  then  the  whole  of  the  heat 
values  of  the  foods  expressed  in  the  last  column,  less 
what  is  stored  as  fat  and  flesh,  will  be  developed  in  the 
animal's  body,  and  will  go  to  keep  it  warm.  If,  however, 
the  animal  is  at  work,  then  the  amount  of  energy  thus 
developed  will  not  appear  as  heat  inside  the  animal.  It 
is,  of  course,  true  that  even  when  at  rest  in  the  stall  the 
animal  is  doing  a  certain  amount  of  internal  work,  but 
this  internal  work  being  done  inside  the  body  is  there 
reconverted  into  heat  without  loss ;  it  is  only  work 
outside  the  body,  as  in  drawing  a  cart  or  turning  a  mill, 
that  is  lost  as  animal  heat,  because  it  is  converted  into 
some  other  form  of  energy  that  can  be  stored  outside 
the   body.     But   as  regards  the   internal   work   of  the 


I90    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

body,  we  must  make  a  distinction  between  the  work 
done  in  carrying  on  such  movements  as  breathing,  the 
circulation  of  the  blood,  etc.,  which  are  bound  up  with 
the  life  of  the  animal,  and  the  other  work  of  digestion — 
mastication,  swallowing,  the  motions  of  the  stomach  and 
intestines — which  will  vary  in  amount  with  the  nature 
of  the  food  supplied.  Pure  fats  and  carbohydrates  like 
sugar  can  be  digested  with  a  minimum  of  exertion,  but 
when  the  same  substances  are  found  in  food-stuffs 
entangled  among  the  fibres  of  tough  materials  like  hay 
and  straw,  the  animal  may  have  to  do  a  good  deal  of 
work  in  breaking  down  the  food  before  the  enzymes  can 
get  at  the  digestible  constituents,  and  this  work  has  to 
be  derived  from  the  combustion  of  some  previously 
digested  food  stored  up  in  the  body,  i.e.  eventually 
from  the  energy  contained  in  the  food  itself.  If,  then, 
for  any  food-stuff  we  begin  by  estimating  a  certain 
number  of  calories  as  its  heat  value,  which  heat  value 
is  the  measure  of  the  energy  it  can  liberate  in  the  body, 
we  shall  have  to  make  a  deduction  for  the  energy  used 
up  in  digestion  before  we  can  get  at  the  energy  remain- 
ing that  is  available  for  work  or  for  such  purposes  as 
putting  on  fat.  If,  indeed,  the  work  spent  in  the 
digestion  of  a  given  food  is  very  large,  it  may  approach 
or  even  exceed  its  total  heat  value,  and  so  leave  no 
margin  for  either  the  internal  or  external  work  of  the 
body.  Of  course,  the  energy  thus  spent  either  in 
digestion  or  internal  work  is  still  transformed  into  heat, 
whereby  the  animal  is  kept  warm  ;  hence  an  animal  at 
rest  on  a  maintenance  diet,  the  heat  value  of  which  just 
supplies  enough  energy  for  both  digestion  and  internal 
work,  will  still  maintain  its  animal  heat,  and  will 
even  be  able  to  increase  it  by  a  greater  consumption  of 
food  if  it  is  forced  to  make  up  for  greater  losses  of 
heat  by  being  put  to  live  under  colder  conditions.     But 


X.]  WORK  EXPENDED  IN  DIGESTION  191 

if  all  the  energy  derivable  from  the  food  is  spent  in 
effecting  digestion  and  internal  work,  there  will  be  no 
margin  left  either  for  external  work  or  the  production  of 
increased  weight,  hence,  however  much  food  the  animal 
ate,  it  can  never  do  any  work  nor  grow  any  heavier. 
This  condition,  when  the  energy  derived  from  the  food 
is  wholly  or  even  more  than  balanced  by  the  energy 
required  for  digestion,  is  realised  in  the  case  of  a  horse 
feeding  upon  straw,  which  is  very  fibrous,  so  that  the 
work  required  is  great  and  a  large  proportion  remains 
undigested,  while  of  the  digested  carbohydrate  about 
one-fifth  is  lost  as  methane.  Zuntz  found  that  the 
horse  actually  consumes  more  energy  in  digesting 
straw  than  is  contained  in  the  portion  digested,  and 
this  is  confirmed  by  an  experiment  of  Miintz,  who  fed  a 
horse  on  straw  alone.  Although  the  horse  was  allowed 
an  unlimited  amount  of  straw  it  died  at  the  end  of  about 
two  months,  thoroughly  exhausted  because  it  had  been 
compelled  to  draw  upon  its  body.  Again,  Kellner,  in 
experiments  with  oxen,  which  are  better  able  than 
horses  to  deal  with  foods  like  straw,  found  that  more 
than  four-fifths  of  the  energy  contained  in  the  digested 
part  of  the  straw  was  consumed  in  the  digestion 
processes,  leaving  less  than  one-fifth  available  for  work 
or  increase.  When,  however,  the  straw  was  made  into 
a  pulp  by  the  processes  employed  by  papermakers, 
who  disintegrate  the  cellulose  by  boiling  with  an  alkali 
under  pressure,  as  much  as  88  per  cent,  of  the  straw  was 
digested,  and  of  this  digestible  matter  a  little  more  than 
a  third  only  was  consumed  in  effecting  digestion. 

So  fundamental  are  these  considerations  regarding 
the  total  and  available  energy  of  foods,  that  we  may 
recapitulate :  the  total  energy  resident  in  any  food  is 
measured  by  the  heat  it  will  evolve  on  burning,  and  is 
called  its  fuel   value.     From   this   fuel   value   must   be 


192    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

deducted  the  fuel  value  of  the  undigested  excreta,  and  also 
of  the  incompletely  oxidised  urea  and  the  gases  evolved  in 
the  intestine,  in  order  to  obtain  the  energy  available  for 
the  animal — the  heat  value  of  the  food.  All  this  energy  is 
available  for  the  maintenance  of  the  heat  of  the  animal, 
but  a  certain  proportion  is  spent  in  the  work  of  digestion, 
and  it  is  only  the  surplus  that  is  available  for  the  internal 
and  external  work  performed  by  the  animal  or  for  the 
increase  of  weight  that  it  may  be  putting  on.  If  the 
food  is  being  added  to  the  maintenance  ration,  all  the 
surplus  will  be  available  for  external  work  or  for 
increased  weight.  We  can  now  define  the  surplus 
energy  which  a  food  will  give  out  over  and  above  the 
work  required  for  its  own  digestion,  as  the  "  dynamic  " 
energy  of  the  food,  and  it  is  this  dynamic  energy  alone 
which  is  available  for  performing  work  or  for  giving 
rise  to  increased  weight.  Thus  we  distinguish  the  total 
energy  or  fuel  value  of  the  food,  the  heat  value  or 
thermal  energy,  and  now  the  dynamic  energy  or  value 
for  work  and  production. 

One  or  two  examples  may  perhaps  make  this  more 
clear. 

Taking  decorticated  cotton  cake  we  may  on  its 
analysis  calculate  the  "  fuel  value  "  from  the  experiment- 
ally determined  facts  that  i  gramme  of  oil  (in  such  seed 
cakes)  produces  9-2  calories  on  combustion,  i  gramme 
of  protein  and  i  of  carbohydrate  producing  5-8  and 
4- 1  calories  respectively.  From  100  grammes  of  decorti- 
cated cotton  cake  we  therefore  obtain  a  fuel  value  of 
460  calories  as  follows  : — 

Oil,  9  grammes  x  9*2  calories       .  .     =       82*8 

Protein,  41  grammes  x  5-8  calories  .     =     237*8 

Carbohydrates  and  Fibre,  34  grammes 

X  4-1  calories  .  .  .  .     =     139-4 

460-0 


X.]  ENERG  V  A  VAILABLE  FOR  WORK  193 

Deductions  must  now  be  made  for  the  undigested 
part  of  the  food,  for  the  urea  and  the  methane  excreted, 
as  follows : — 

Undigested  Oil,  0-5  grammes  at  9-2  cals.  .  =  4*6 
Undigested  Protein,  7  grammes  at  5-8  cals.  .  =  40-6 
Undigested  Fibre,  14  grammes  at  4-1  cals.  =       57*4 

Urea  from  34  grammes  digested  protein  at 

I- 1  calories      .  .  .  .  .     =       37'4 

Methane  from   20  grammes   digested  carbo- 
hydrates, etc.,  at  0-4  calories  .  .     =         8-o 

148-0 

Deducting  these  148  calories  from  the  fuel  value, 
460  calories,  we  get  312  calories  as  the  heat  value  of  the 
cotton  cake.  We  have  now  to  make  a  further  deduc- 
tion of  3  per  cent,  (or  9  calories)  as  the  internal  work 
spent  in  digestion,  leaving  303  calories  as  the  dynamic 
energy  of  100  grammes  of  decorticated  cotton  cake  avail- 
able for  transformation  into  work  or  storage  as  fat  in  the 
animal ;  though,  as  will  be  seen  later,  only  a  certain  frac- 
tion of  this  can  be  so  converted  or  stored. 

Let  us  now  compare  meadow  hay,  calculating  on  the 
same  lines : — 

2-5  grammes  fat  at  8«2  calories  .  .     =       20*5 

10  grammes  protein  at  5-8  calories       .  .     =       58-0 

68  grammes  carbohydrates  and  fibre  at  4-1  cals.  =     278'8 

Calories,  fuel  value  357*3 

Deductions : — 

Undigested  Fat,  i'5  grammes  at  8-2  calories  .  =  I35'3 
Undigested  Protein,  6  grammes  at  5-8  cals.  .  =  34-8 
Undigested  Fibre,  27  grammes  at  4' I  cals.  .  =  1107 
Urea  from  4  grammes  digested  protein,  at 

M  calories  .  .  .  ,     =         4*4 

Methane  from  41  grammes  digested  carbo- 
hydrates, etc.,  at  0-4  calories  .  .     =       i6'4 


178-6 


N 


194    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

Deducting  this  from  the  fuel  value  we  get  357-3  — 
178-6=  178-7  calories  as  the  heat  value  of  100  grammes 
of  meadow  hay.  But  from  this  we  have  to  deduct  40  per 
cent,  for  internal  work  spent  in  digestion,  or  71-5 
calories,  leaving  only  107-2  calories  as  the  dynamic  energy 
of  100  grammes  of  meadow  hay.  Thus  the  meadow 
hay,  while  it  possesses  more  than  half  (178  compared 
with  312)  the  heat  value  of  the  cotton  cake — i.e,  is  more 
than  one-half  as  effective  in  keeping  up  the  heat  of  the 
animal  on  a  maintenance  ration — has  yet  only  about 
one-third  of  the  value  of  cotton  cake  (107  as  against  303) 
towards  doing  work  or  putting  on  increased  weight. 

Bearing  these  principles  in  mind,  we  may  trace 
certain  simple  practical  consequences.  Rough,  coarse 
fodders  like  hay  or  straw,  poor  grass,  roots,  etc.,  serve 
perfectly  well  for  keeping  animals  in  store  condition,  for 
though  they  require  a  considerable  expenditure  of  energy 
for  their  digestion  there  is  enough  margin  to  carry  on 
the  internal  work  of  the  body  and  the  whole  energy  is 
afterwards  available  as  heat,  while  the  animal  has  only 
to  be  kept  warm  and  is  neither  working  nor  increasing 
in  weight.  Similarly,  animals  that  are  at  slow  work  and 
are  never  called  upon  for  any  great  output  of  energy  in 
proportion  to  their  weight,  can  be  fed  upon  bulky  low- 
grade  fodders  which  do  not  develop  any  great  surplus 
of  energy.  But  when  animals  are  growing  rapidly  or 
are  performing  heavy  and  rapid  work,  then  comparatively 
rich  and  concentrated  foods  are  necessary,  foods  which 
develop  a  large  surplus  of  energy  over  that  which  is 
required  for  their  digestion.  A  horse  standing  in  the 
stable  may  be  fed  on  nothing  but  hay,  just  as  a  horse 
out  at  grass  needs  only  a  little  hay  besides  the  old  grass 
even  in  severe  winters,  but  as  soon  as  the  horse  is 
worked,  instead  of  more  hay  it  must  be  given  corn  of 
some  kind ;  and  a  racehorse,  on  which  great  calls  are 


X.]  VALUE  OF  CONCENTRATED  FOODS  195 

made  for  a  sudden  and  excessive  output  of  energy,  must 
have  the  most  concentrated  and  digestible  foods  that 
can  be  obtained.  No  increase  in  the  amount  of  the 
lower-grade  foods  will  compensate  for  their  lack  of 
concentration,  because  so  much  time  would  be  spent  by 
the  animal  in  heaping  up  the  necessary  surplus  energy. 
To  take  another  example,  cattle  or  sheep  will  never 
grow  fat  in  one  season  on  the  grass  growing  on  the 
majority  of  fields,  however  great  an  area  they  may  be 
given  to  graze ;  it  is  only  on  certain  choice  fattening 
pastures  that  the  increase  is  rapid  enough  to  prepare  an 
animal  for  market  without  artificial  assistance.  On  the 
ordinary  grass  lands  the  animal  spends  so  much  of  the 
energy  obtained  from  its  food  in  digesting  it  that  the 
surplus  left  for  production  does  not  permit  of  rapid 
growth ;  on  the  fattening  fields  the  grass  possesses  a 
smaller  proportion  of  fibre,  and  therefore  less  of  its  heat 
value  is  wasted  in  the  digestion  processes. 

Similarly,  in  the  last  stages  of  fattening  animals  in 
stalls  carbohydrates  are  of  much  less  value  than  they  are 
at  an  earlier  period,  because  they  call  for  an  expenditure 
of  energy  in  digestion  which  is  disproportionate  to  the 
increase  they  produce  at  that  stage,  when  the  increase 
bears  a  very  small  ratio  to  the  food  consumed,  whereas 
much  less  of  the  energy  of  fats  and  proteins  is  wasted 
in  the  digestion  process. 

Treating  food  from  this  point  of  view  as  supplying 
energy  to  the  animal,  we  may  now  proceed  to  consider 
the  animal's  requirements  under  different  conditions. 
The  simplest  case  is,  of  course,  that  of  the  animal  at 
rest  on  a  maintenance  diet,  so  that  it  is  neither  increasing 
nor  diminishing  in  weight.  As  the  temperature  of  the 
animal  is  always  higher  than  the  surrounding  atmo- 
sphere (from  100°  to  104°  R,  varying  with  the  animal), 
a  constant  loss  of  heat  is  going  on  from  the  surface  of 


196    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

the  body,  and  the  food  supply  must  be  sufficient  to 
repair  this  loss  or  the  animal  will  be  forced  to  consume 
some  of  its  previously  stored-up  fat  or  flesh.  All 
internal  work,  including  that  done  during  digestion, 
reappears  as  heat,  so  that  as  long  as  the  food  is 
above  the  grade  of  straw  and  contains  more  available 
energy  than  is  required  for  the  digestive  process,  the 
food  is  only  called  upon  to  repair  the  losses  of  heat  and 
maintain  the  body  temperature ;  the  work  required  for 
digestion,  respiration,  and  other  bodily  processes  being 
performed  by  the  way  without  interfering  with  the  final 
production  of  heat,  because  it  is  work  done  inside  the 
animal.  In  other  words,  the  maintenance  diet  con- 
tains just  enough  energy  to  drive  the  machine  when  it  is 
running  idly  with  the  animal  at  rest,  and  in  the  running 
of  the  machine  the  food  energy  is  transformed  into  heat 
which  keeps  up  the  body  temperature.  Actually  on 
most  diets,  even  maintenance  rations,  the  energy  re- 
quired by  the  resting  animal  to  maintain  its  temperature 
is  greater  than  that  required  for  internal  work ;  some  of 
the  food  is  burnt  simply  and  solely  as  fuel  for  warming, 
and  the  amount  of  food  required  will  be  determined 
only  by  the  heat  lost  by  the  animal.  The  magnitude 
of  this  factor  will  vary  with  the  size  of  the  animal,  or 
rather  with  the  surface  it  possesses,  because  heat  is  lost 
only  from  the  surface.  Now  the  smaller  the  animal  the 
greater  is  its  surface  in  proportion  to  its  weight  (a  cubic 
foot  of  water  weighs  62-5  lb.  and  has  a  surface  of  6 
square  feet ;  a  cube  2  feet  on  the  side  would  weigh  eight 
times  as  much,  but  only  have  a  surface  of  24  square  feet, 
i.e.  four  times  as  much  as  before) ;  hence  small  animals 
will  require  for  maintenance  more  food  in  proportion  to 
their  weight  than  large  ones.  Of  course,  the  greater  the 
difference  in  temperature  between  the  animal  and  its 
surrounding  air  the  greater  will  be  the  loss  of  heat  and 


X.]  ENERG  V  REQUIRED  B  V  ANIMALS  197 

the  consumption  of  food  to  repair  the  loss ;  hence  the 
truth  of  the  old  saying  that  shelter  is  as  good  as  a  meal. 
Heat  may  also  be  required,  and,  therefore,  food  consumed 
in  raising  the  temperature  of  the  food  and  water  to  the 
body  temperature,  and  this  may  be  considerable  when 
large  quantities  of  very  cold  water,  or  roots  which 
contain  nearly  90  per  cent,  of  water  are  consumed 
at  a  freezing  temperature.  This,  however,  would  only 
affect  animals  on  a  maintenance  ration  when  the  food 
is  reduced  to  the  minimum  necessary  to  keep  the 
animals  warm  ;  on  fattening  rations  there  is  always  a 
surplus  of  heat  that  cannot  be  utilised  in  any  other  way. 
Leaving,  however,  such  cases  out  of  account,  Kellner 
has  drawn  up  the  table  from  which  the  diagram.  Fig. 
23,  has  been  constructed,  showing  the  heat  value  of 
the  food  required  for  the  maintenance  of  store  bullocks 
of  various  weights  when  they  are  kept  at  rest  at  a 
temperature  of  about  60°  F.  The  solid  line  expresses  the 
number  of  calories  which  the  digestible  part  of  the  food 
must  give  out  per  day  in  order  to  keep  the  animal  in  a 
stationary  condition ;  these  calories  can  be  converted 
into  terms  of  food  by  calculating  that  i  lb.  of  digestible 
organic  matter  in  an  ordinary  ration  will  evolve  about 
1600  calories,  and  i  lb.  of  starch  about  1 700  calories  ;  the 
dotted  line  gives  the  equivalent  in  starch  of  the  solid 
line.  A  fat  bullock  weighing  about  1750  lb.  requires 
about  20,000  calories  heat  value  in  its  daily  food  in 
order  to  keep  it  going.  The  maintenance  requirements 
of  a  horse  are  very  similar  to  those  of  a  bullock  ;'  accord- 
ing to  Zuntz,  the  maintenance  ration  of  a  horse  at  rest 
must  evolve  about  12,100  calories  per  diem,  and  not 
more  than  two-thirds  of  this  energy  must  be  expended 
in  the  work  of  digestion.  Sheep  have  rather  greater 
requirements  in  proportion  to  their  weight,  because  of 
their   smaller   size   and    therefore   larger    proportional 


198    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

surface,  and  also  because  of  their  higher  temperature; 
a  sheep  weighing  lOO  lb.  requires  digestible  food  which 
will  develop  about  2000  calories,  ix.  1000  lb.  of  sheep 
requires  20,000  calories,  whereas  1000  lb.  of  lean  cattle 
only  require  about  11,000  calories.  All  these  figures, 
however,  refer  only  to  maintenance  diets,  when  the 
animal  is  doing  no  work  and  not  putting  on  flesh.  As 
soon  as  work  has  to  be  performed  the  number  of  calories 
required  jumps  up  :  for  example,  a  horse  weighing  iioo 
lb.,  and  carrying  harness  weighing  about  20  lb.,  will 
perform  about  11 50  foot-tons  of  work  in  walking  10 
miles  at  a  pace  of  2\  miles  an  hour ;  this  will  be 
increased  to  about  1440  foot-tons  at  l\  miles  an  hour, 
and  to  about  2200  foot-tons  when  trotting  at  7  miles  an 
hour.  Foot  -  tons  can  be  transformed  into  calories 
directly  on  the  basis  that  1-4  foot-tons  of  work  will 
yield  i  calorie  when  degraded  into  heat  by  friction, 
but  the  converse  change  cannot  be  made  so  readily, 
because  a  large  proportion  of  the  heat  must  always  be 
left  unutilised  in  its  conversion  into  work.  A  steam 
engine,  for  example,  is  a  machine  for  transforming  heat 
energy  into  mechanical  energy,  yet  at  its  utmost 
efficiency  we  can  barely  get  out  in  work  one-seventh  of 
the  energy  contained  in  the  coal,  the  rest  being  evolved 
in  waste,  low-grade  heat  From  this  point  of  view  the 
animal  is  a  much  more  efficient  machine  than  a  steam 
engine,  for  it  is  able  to  convert  into  mechanical  work 
about  one-third  of  the  available  energy  it  derives  from 
its  food,  i.e.  of  the  "dynamic"  energy  of  the  food. 
According  to  Zuntz's  experiments,  a  horse  can  turn  out 
about  770  foot- tons  of  work  for  each  pound  of  available 
food,  reckoned  as  usual  as  starch,  given  in  addition  to 
the  animal's  maintenance  diet — the  available  food  being, 
however,  the  equivalent  of  what  we  have  hitherto  called 
dynamic  energy,  or  the  value  of  the  food  for  work,  i.e. 


Heat 

Value 
of  Food 

Calories. 

20,000 


15,000 


Starch 
Equiva- 
lent. 

iLb; 


Live  weight, 

lb.         900 


^ 

H 

ej:J 

alU^^ 

^fj^ 

D^^^^ 

- 

STAPi^. 

f  0JI 

/AL£N 

r 



looo       iioo        1200        1300        1400 


[500      1600        1700       1800 


Fig.  23.— Diagram  showing  the  Energy  required  from  the  Food 
BY  Oxen  of  Different  Weights. 


[Face  fage  198. 


X.] 


FOOD  REQUIREMENTS  OF  HORSES 


199 


after  deductions  have  been  made  for  energy  spent  in 
digestion,  etc.  As  different  foods  possess  different 
amounts  of  available  energy,  Zuntz  has  constructed  the 
following  table  for  the  horse  : — 

Table  XIX.— Available  Energy  of  Various  Foods. 


Value  as 

Sterch 

of  Digested 

Portion. 

Used  for 

Digestion 

Work,  as 

Starch. 

Foot-tons  of 
Work  avail- 
able per  lb. 
of  Food. 

Equivalent 

Quantities  of 

Food  for 

Work 
Purposes. 

Starch     . 
Maize     . 
Beans      . 
Oats 

Lucerne  Hay 
Meadow  Hay 
Straw      . 

ICX) 

78-5 

72-0 

61.5 
45-3 
39-1 
18.1 

0 

8-2 
II-I 
12.4 
21.9 
20-0 
29.7 

542 
470 

379 
180 
140 

Nil. 

10 

I-I5 

1-43 

3-OI 

3-87 

This  table  means  that  the  digestible  portion  of  a 
pound  of  maize  has  a  heat  value  to  the  horse  equal  to 
that  of  rather  more  than  three-quarters  of  a  pound  of 
starch;  nearly  10  per  cent,  of  this  heat  value  is  used  up 
in  the  work  of  digesting  the  maize,  but  from  the 
available  energy  remaining  the  horse  can  put  out  542 
foot-tons  of  work.  On  meadow  hay,  however,  although 
it  contains  just  half  the  quantity  of  digestible  food,  yet 
so  much  energy  is  consumed  in  digestion  that  the 
balance  only  enables  the  horse  to  put  out  140  foot-tons 
of  work,  or  only  one-quarter  the  work  that  can  be  done 
on  the  maize,  though  the  food  yields  half  as  much 
digestible  matter.  In  the  case  of  straw  there  is  no 
margin  at  all  left  for  work.  We  also  see  that  if  the 
addition  of  i  lb.  of  maize  to  a  horse's  diet  enables  him 
to  do  a  certain  amount  of  work,  1-43  lb.  of  oats  or  3  lb. 
of  lucerne  hay  would  be  necessary  to  replace  the  maize 
and  turn  out  the  same  amount  of  work.  Similarly,  it  was 
found  that  when  a  horse  walked   12^  miles  a  day,  20  lb. 


200    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

of  hay  would  keep  it  in  condition  ;  if  it  drew  a  load  that 
added  1943  foot-tons  to  its  work,  26-4  lb.  of  hay  were 
needed  ;  while  24  lb.  were  insufficient  if  the  horse  trotted 
without  a  load.  The  load  meant  6-4  lb.  extra  of  hay, 
and  according  to  our  table  a  better  effect  would  have 
been  produced  by  substituting  2  lb.  of  maize. 

It  is  difficult  as  yet  to  apply  these  kind  of  calcu- 
lations to  practice  because  of  the  impossibility  of 
estimating  with  exactitude  the  amount  of  work 
performed  by  a  horse  in  any  operation,  but  we  may 
assume  that  at  ordinary  heavy  work  like  ploughing,  a 
horse  will  be  doing  about  1000  to  iioo  foot-tons  of  work 
per  hour.  To  do  this  work,  2  lb.  of  maize,  2\  lb.  of 
beans,  or  nearly  3  lb.  of  oats  will  be  required ;  or  for  an 
eight-hour  day's  work,  about  20  lb.  of  mixed  corn.  To 
this  must  be  added,  not  the  whole  maintenance  require- 
ment of  the  horse,  because  his  heat  requirements  will  be 
satisfied  by  the  development  of  heat  from  the  muscular 
work  spent  in  the  body  and  in  digestion,  but  instead 
about  a  third  of  the  whole  amount  in  order  to  do  the 
work  of  respiration  and  circulation,  i.e.  about  3  lb.  more 
of  corn.  If  some  of  this  corn  is  replaced  by  its 
equivalent  in  hay  we  get  a  fair  approximation  to  the 
ordinary  working  rations  of  a  farm  horse,  and  these  can 
be  adjusted  on  the  principle  that  every  hour's  work  is 
equivalent  to  between  2  lb.  and  3  lb.  of  corn.  These 
figures  are  contained  more  exactly  in  the  following  table, 
derived  from  Kellner,  where  the  horse  is  made  to  work 
under  experimental  conditions,  walking  at  the  rate  of 
about  2\  miles  per  hour,  against  the  draught  indicated, 
for  eight  hours  a  day. 

The  concentrated  foods  like  oats,  beans,  or  maize  yield 
very  nearly  their  full  value  in  starch  equivalents  for  all 
the  digestible  constituents  of  the  food,  i.e.  nearly  all  the 
digestible  constituents  are  available  for  work.     But,  as 


X.] 


HORSE  RATIONS 


20I 


we  have  explained  before,  with  the  coarse  fodders  much 
of  the  energy  is  spent  in  the  work  of  digestion,  so  that 
only  a  portion  is  available  for  work,  though  all  the  heat 
value  is  available  for  maintenance.  In  the  ordinary 
horse  ration,  we  may  deduct  about  lo  per  cent,  from  the 
digestible  food  constituents  reckoned  as  starch  in  order 
to  obtain  figures  comparable  to  those  given  in  the 
bottom  line  of  the  table.  In  a  later  chapter  more  exact 
figures  will  be  given  for  the  starch  equivalent. 

Table  XX.— Energy  Requirements  of  Working  Horses. 


Live  weight  of  horse,  lb. 

675 

900 

1125 

1350 

1575 

Draught  in  lb.  weight  . 

lOI 

126 

151 

175 

220 

Daily  work  in  foot-tons 

4200 

6000 

7200 

8400 

9600 

Calories  required  for  maintenance 

9000 

10900 

12600 

14300 

15800 

„              „        for  work    . 

10400 

12900 

15500 

18000 

20600 

Starch    equivalent    for     mainten- 

ance, lb 

5-3 

6.4 

7-4 

8.4 

9-3 

Starch  equivalent  for  work,  lb.     . 
Total  Starch  equivalent,  lb. 

6-1 

7-6 

9.1 

10-6 

I2-I 

1 1.4 

14-0 

16.5 

190 

21-4 

So  far,  the  food  has  only  been  considered  as  a  source 
of  energy,  and  this  is  much  its  most  important  function, 
so  that  the  best  measure  of  the  value  of  a  food  is  the 
amount  of  available  energy  it  possesses.  Food,  how- 
ever, has  also  to  effect  the  renewal  of  the  tissues  of  the 
body,  and  this  function  must  be  considered  indepen- 
dently of  the  supply  of  energy. 

An  animal  cannot  be  maintained  without  a  certain 
minimal  supply  of  protein  containing  nitrogen,  and 
though  the  protein  can  supply  energy  and  when  in 
excess  can  even  give  rise  to  fat,  its  prime  purpose  is  to 
repair  the  nitrogenous  waste  of  the  tissues,  and  for  this 
no  other  nutrient  will  suffice.  Proteins  give  off  heat 
when  burnt  in  the  body  and  so  can  take  the  place 
of  fats  or   carbohydrates,  but  as  these  two  latter  con- 


202    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

tain  no  nitrogen  they  cannot  do  the  special  work  of 
the  proteins.  We  have  already  briefly  indicated 
the  metabolism  suffered  by  the  proteins  in  the 
diet;  they  are  broken  down  by  the  enzymes  of  the 
stomach  and  the  intestine  into  comparatively  simple 
amino-acids  and  amides,  and  in  this  form  are  ab- 
sorbed by  the  walls  of  the  stomach  and  intestines. 
It  is  still  in  doubt  whether  they  are  there  recon- 
verted into  proteins  which  pass  into  the  blood,  or 
whether  the  blood  is  given  the  simple  split  products 
resulting  from  the  enzyme  action.  The  end  result  is 
the  same  in  either  case,  the  nitrogenous  compounds  are 
carried  always  in  the  blood-stream  and  so  reach  succes- 
sively every  cell  in  the  body  where  such  materials .  are 
absorbed  as  are  required  to  maintain  the  structure  of 
the  cell ;  at  the  same  time  the  cell  hands  over  to  the 
blood  the  waste  nitrogen  products  excreted  by  the  cells. 
The  amount  of  nitrogen  compounds  thus  taken  up  for 
tissue  repairs  and  renewals  is  only  a  small  portion  of 
the  nitrogenous  compounds  entering  the  blood  from  the 
food,  unless  the  animal  has  been  placed  on  a  minimal 
ration  which  barely  maintains  a  nitrogen  equilibrium. 
As  a  rule,  the  animal  is  receiving  more  protein  than  is 
absolutely  necessary,  but  when  the  blood  containing  the 
digested  protein  products  passes  through  the  kidneys 
the  nitrogen  part  is  split  off  in  the  form  of  urea  and 
only  non-nitrogenous  compounds  are  passed  on,  to 
be  used  for  generating  energy  or  forming  fat  as  need 
may  be.  This  accounts  for  the  rapid  excretion  of  urea 
after  a  meal,  when  it  cannot  be  supposed  that  time 
enough  has  elapsed  to  give  rise  to  an  amount  of  tissue 
waste  equivalent  to  the  urea  excreted.  Thus  we  must 
distinguish  between  the  excreted  nitrogen  compounds, 
which  are  due  to  tissue  waste  and  represent  the 
indispensable   nitrogen    requirements   of    the   body   if 


X.]  PROTEIN  REQUIRED  BY  ANIMALS  203 

its  equilibrium  is  to  be  maintained,  and  the  other 
nitrogen  compounds  (urea  only,  probably)  split  off 
from  proteins  which  are  being  used  as  mere  sources 
of  energy,  though  both  kinds  are  excreted  together 
in  the  urine.  The  former  quantity  of  nitrogen,  the 
minimum  required  to  repair  tissue  waste,  can  be 
measured  either  by  putting  the  animal  on  a  purely  non- 
nitrogenous  diet  and  seeing  how  much  nitrogen  is 
excreted  under  such  conditions  of  nitrogen  starvation, 
or,  more  accurately,  by  gradually  reducing  the  protein  in 
a  ration  supplying  the  proper  amount  of  energy  until  a 
limiting  condition  of  nitrogen  equilibrium  is  reached 
when  any  further  diminution  in  the  protein  fed  results 
in  the  excretion  remaining  greater  than  the  intake, 
after  the  first  disturbance  due  to  the  change  of  diet  has 
passed  away.  In  this  way  the  minimum  requirements 
of  protein  for  oxen  or  horses  on  a  maintenance  diet 
appear  to  be  about  \  lb.  per  day  per  1000  lb.  live 
weight ;  for  sheep  this  quantity  must  be  nearly  doubled, 
because  of  the  comparatively  large  draught  on  protein 
for  the  growth  of  wool,  which  cannot  be  brought  to  a 
standstill  like  the  formation  of  flesh.  It  is,  however, 
never  safe  in  feeding  animals  to  get  down  to  this 
minimal  limit ;  the  digestibility  of  the  carbohydrates  in 
the  ration  is  reduced  if  the  proteins  are  low,  especially 
when  the  rations  are  large,  because  the  animal  is  then 
called  upon  to  secrete  an  increased  amount  of  enzymes 
which  are  themselves  nitrogenous  bodies.  Moreover,  it 
has  been  shown  that  respiration  is  quickened,  the 
circulation  is  more  vigorous,  and  the  temperature  is 
raised  a  trifle,  if  the  supply  of  proteins  is  above  the 
absolute  minimum.  There  is  another  point  of  view 
also  to  be  considered  :  though  the  proteins  form  a  great 
natural  group  of  substances  having  many  characters 
in  common,  which  can  be  further  grouped  into  smaller 


204    UTILISATION  OF  FOOD  BY  THE  ANIMAL  [chap. 

classes  of  still  more  similar  substances,  yet  it  is  probable 
that  each  species  of  animal  and  plant  builds  up  a  protein 
special  to  itself  and  differing  somewhat  from  all  others. 
In  order  to  build  up  its  special  body  protein  an  animal 
must  be  supplied  with  the  right  units,  />.,  the  simple 
amino-acids,  amides,  etc.,  which  are  produced  by  the 
splitting-up  of  proteins  by  enzymes,  and  those  units 
which  the  animal  does  not  require  will  be  useless  to  it  as 
nitrogenous  food,  though  they  will  be  burnt  as  usual  to 
supply  energy.  Some  food  proteins  also  may  be  with- 
out a  unit  that  is  essential  to  the  building-up  of  the 
body  protein  ;  thus  it  has  been  shown  that  zein,  the 
particular  protein  of  maize,  will  not  maintain  the 
nitrogen  equilibrium  of  rats  or  mice.  However,  much 
of  it  is  fed,  they  eventually  die  of  nitrogen  starvation, 
because  the  zein  molecule  lacks  a  particular  constituent 
which  is  an  essential  part  of  the  body  protein  of  these 
animals.  If,  however,  a  very  small  quantity  of  this 
particular  substance  is  also  supplied,  then  the  animal 
begins  to  utilise  the  zein  split  products  and  form  its 
necessary  body  protein,  because  it  has  also  obtained  the 
necessary  keystone  of  the  structure.  Thus  we  see  that 
the  animal  must  have  more  products  of  protein  digestion 
presented  to  it  than  it  actually  uses,  because  it  has  to 
pick  them  over  in  order  to  select  the  indispensables  in 
the  right  proportions ;  moreover,  the  diet  should  contain 
some  variety  in  the  proteins  supplied,  in  order  to  make 
certain  that  no  essential  constituent  is  deficient. 

The  food  of  an  animal  not  increasing  in  weight  should 
therefore  be  considered  from  two  points  of  view — it 
must  supply  sufficient  energy  for  the  calls  upon  the 
animal  for  work,  and  at  the  same  time  it  must  contain 
sufficient  protein  to  repair  the  tissue  waste  that  an 
animal  always  experiences  even  during  starvation. 
Energy  is  supplied  by  all  kinds  of  food  that  are  com- 


X.]  ALBUMINOID  RATIO  205 

bustible — most  by  fat,  less  by  proteins,  less  still  by 
carbohydrates  and  crude  fibre  as  far  as  it  is  digestible — 
and  the  amount  of  energy  that  must  be  supplied  by  the 
food  depends  simply  upon  the  amount  of  work  done  by 
the  animal.  In  animals  at  rest  energy  is  only  required 
to  perform  the  operations  of  mastication  and  digestion, 
internal  work  like  breathing,  and  to  maintain  the 
temperature  of  the  body,  though  the  heat  developed  in 
performing  the  former  functions  may  be  sufficient  to 
keep  up  the  bodily  heat.  When  an  animal  is  at  work 
combustion  is  going  on  so  rapidly  that  far  more  heat  is 
developed  than  is  necessary  to  the  maintaintence  of  the 
normal  temperature,  and  the  excess  is  got  rid  of  by  an 
increased  evaporation  of  water  from  the  lungs  and  the 
skin,  for  the  animal's  temperature  never  rises  sensibly 
as  long  as  it  is  healthy.  In  addition  to  food  supplying 
the  necessary  energy,  the  animal  must  also  receive  at 
least  a  minimal  amount  of  protein  which  it  can  utilise 
for  tissue  formation  ;  any  excess  is  rapidly  reduced  to 
non-nitrogenous  combinations  and  then  burnt  up  for  the 
development  of  energy  like  any  other  food. 

From  the  point  of  view  of  the  food,  we  must  consider 
that  it  contains  first  of  all  a  certain  store  of  energy 
measured  by  its  value  as  fuel,  from  which  to  begin  with 
we  must  deduct  the  energy  of  the  indigestible  portions, 
and  the  energy  still  possessed  by  excreta  like  urea  and 
marsh-gas,  in  order  to  get  the  heat  value  of  the  food  to 
the  animal.  Further  still,  we  must  deduct  from  this 
value  the  work  spent  by  the  animal  in  masticating  and 
digesting  the  food  if  we  are  to  obtain  the  energy  value 
of  the  food  to  the  animal — i.e,  the  surplus  remaining  out 
of  which  it  can  do  work.  We  now  want  an  expression  for 
the  protein  value  of  the  food,  and  to  do  this  we  are 
accustomed  to  calculate  the  ratio  between  the  non- 
nitrogenous  food  constituents  and  the  protein.     To  make 


2o6  UTILISATION  OF  FOOD  B  V  THE  ANIMAL  [chap.  x. 

the  comparison  fair,  the  non-nitrogenous  food  con- 
stituents must  be  reckoned  in  terms  of  carbohydrates 
by  multiplying  the  fat  by  2-3  and  adding  it  to  the 
carbohydrates  and  digestible  fibre ;  the  ratio  of  the  sum 
of  these  constituents  to  the  digestible  protein  is  known 
as  the  albuminoid  ratio.  If  the  albuminoid  ratio  is 
to  be  a  figure  of  the  slightest  value  in  judging  of  a 
food  ration,  it  must  be  calculated  on  the  digestible 
constituents  only  and  not  on  the  analytical  composition 
simply,  as  is  so  often  done.  An  albuminoid  ratio  of  5  :  i  or 
less  is  said  to  be  narrow ;  when  it  approaches  10 :  i  or 
12:1  it  is  called  wide.  The  ratio  is  of  comparatively 
little  importance  in  connection  with  the  cases  we  have 
just  been  discussing — animals  at  work  and  not  increasing 
in  weight ;  but  when  we  are  dealing  with  animals  which 
are  growing  and  increasing  in  weight,  which,  therefore, 
especially  in  the  younger  stages,  are  adding  to  the  amount 
of  body  protein  they  contain,  or  again  with  animals 
which  are  secreting  milk,  the  albuminoid  ratio  becomes 
of  more  value  in  forming  a  judgment  of  a  ration.  This 
most  important  side  of  feeding  will  be  considered  in  the 
following  chapter. 


CHAPTER   XI 

FOOD  REQUIRED  BY  THE  GROWING  AND 
FATTENING  ANIMAL 

Composition  of  Lean  and  Fat  Animals.  Food  required  to  Produce 
a  given  Increase  of  Live  Weight.  Starch  Values  of  Foods. 
Albuminoid  Ratio.  Food  Rations  for  various  Purposes 
based  upon  Starch  Values. 

In  the  previous  chapter  we  have  only  considered  food 
as  a  source  of  energy  to  the  animal,  maintaining  its 
temperature,  and  then  supplying  it  with  energy  by 
which  it  may  carry  out  its  bodily  functions  and  perform 
work.  But  the  farmer  is  usually  concerned  with  animals 
which  are  growing  and  increasing  in  weight,  and  we 
have  now  to  ask  ourselves  to  what  extent  the  food  is 
utilised  in  these  processes.  The  foundation  of  our 
conclusions  must  rest  upon  the  knowledge  of  the 
composition  of  the  animal's  carcass  in  different  stages 
of  its  life,  and  this  knowledge  was  obtained  at  Rotham- 
sted,  where  Lawes  and  Gilbert  had  a  number  of  animals 
slaughtered  in  different  stages  of  fatness,  and  determined 
stage  by  stage  the  composition  of  the  body  of  each. 
The  following  animals  were  selected  : — a  fat  calf,  a  half- 
fat  and  a  fat  ox ;  a  fat  lamb,  a  store  sheep,  three  other 
sheep  in  a  half-fat,  fat,  and  very  fat  condition ;  a  store 
and  a  fat  pig.  The  animals  after  slaughter  were  care- 
fully divided,  and  the  weights  of  the  different  parts  of 
the  carcass  and  the  offal  were  determined ;  afterwards 


2o8 


FOOD  REQUIRED 


[chap. 


the  proportions  of  water,  fat,  protein,  and  ash  in  each 
part  were  determined,  with  the  results  set  out  in  Table 
XXI.  and  further  illustrated  in  Diagram  No.  24.     On 


Table  XXL— Percentage  Composition  of  Carcasses  of 
Animals  in  Different  Stages  of  Fattening. 


i 

li 

bS 

^1g? 

11 

II 

t 

1 

Contents 

tomachs 

Intestin 

Moist  sta 

S 

tn      ^-> 

Fat  Calf 

3-8o 

15-2 

14-8 

33-8 

63-0 

3-17 

Half-fat  Ox    . 

4.66 

16-6 

19.1 

40-3 

51-5 

8-19 

Fat  Ox. 

3-92 

14-5 

30-1 

48.5 

45-5 

5-98 

Fat  Lamb      . 

2.94 

12.3 

28.5 

43-7 

47-8 

8-54 

Store  Sheep   . 

3-16 

14-8 

18.7 

36.7 

57-3 

6-00 

Half-fat  Old  Sheep 

3-17 

I4-0 

23-5 

40.7 

50-2 

9-05 

Fat  Sheep      . 

2.81 

12-2 

35-6 

50-6 

43-4 

6 -02 

Extra  Fat  Sheep    . 

2.90 

10.9 

45-8 

59-6 

35-2 

5-i8 

Store  Pig       .    .     . 

2.67 

137 

23-3 

39-7 

55-1 

5-22 

Fat  Pig . 

Mean  of  all 

1.65 

I0'9 

42.2 

54-7 

41-3 

3-97 

3-17 

13-5 

28.2 

44.9 

49-0 

6.13 

PERCENTAGE  COMP 

OSITIO^ 

r  OP  IN 

CREASI 

]  IN  F^ 

iTTENI 

NG. 

Oxen      .... 

1.67 

9-i6 

75-36 

86-19 

13-81 

Sheep    .... 

2-34 

9-47 

79.87 

91-68 

8-32 

... 

Pigs       .... 

o-o6 

6-50 

78-00 

84.56 

15-44 

... 

looking  at  these  results,  it  will  be  seen  that  the  process 
of  fattening  is  very  much  what  its  name  implies.  As 
the  animal  puts  on  weight,  there  is  little  or  no  increase 
in  the  proportion  of  either  the  ash  or  the  protein,  but 
the  fat  gains  steadily  at  the  expense  of  the  water.  Of 
course,  as  an  animal  is  increasing  in  weight  during 
the  fattening,  the   actual  amount   of  protein  matter  it 


XI.]      COMPOSITION  OF  FATTENING  ANIMALS      209 

contains  does  also  increase,  but  not  so  rapidly  as  the 
fat,  so  that  its  percentage  of  the  total  weight  actually 
falls  off.  It  should  also  be  noticed  that  the  fat  animal 
contains  a  smaller  proportion  of  water  than  the  same 
animal  in  a  store  condition,  lean  meat  is  a  much  more 
watery  substance  than  fat,  so  that  the  accumulation  of 
fat  tends  to  reduce  the  proportion  of  water  in  the  whole 
body.  If  we  can  assume  that  the  different  animals 
slaughtered  in  the  various  stages  of  fatness  fairly 
represent  the  kind  of  changes  that  the  single  animal 
would  have  gone  through  while  it  was  being  fattened, 
we  can  use  these  figures  of  Lawes  and  Gilbert  to 
ascertain  the  composition  of  the  increase  in  live  weight 
which  the  animals  put  on  in  passing  from  the  store  to 
the  half-fat,  and  from  the  half-fat  to  the  fat  condition. 
It  will  then  be  found  that  in  the  case  of  oxen  which  are 
being  steadily  fattened  from  the  time  they  are  young, 
the  increase  of  weight  will  consist  of  about  one-third 
water  and  two-thirds  dry  substance,  about  three-fourths 
of  the  latter  consisting  of  fat.  In  the  final  finishing 
stage,  when  the  animal  is  fully  grown,  about  three-fourths 
of  the  increase  will  be  dry  matter,  90  per  cent,  of  which 
will  consist  of  fat.  In  the  case  of  sheep,  there  is  more 
mineral  matter  in  the  increase  because  of  the  amount  of 
alkaline  salts  in  the  wool ;  but  despite  the  nitrogenous 
nature  of  the  wool,  the  live  weight  increase  of  sheep  is 
even  less  nitrogenous  and  more  fatty  than  that  of  oxen. 
In  fact,  about  75  per  cent,  of  the  increase  of  weight  in 
fattening  sheep  is  made  up  of  fat  itself  In  the  case  of 
heavy  fat  pigs  the  increase  put  on  is  still  less  nitro- 
genous and  more  fatty,  there  being  about  80  per  cent,  of 
fat  and  only  7  per  cent,  of  nitrogenous  matter  in  the 
increase.  While  these  figures  give  us  a  pretty  clear 
idea  of  the  nature  of  the  changes  that  are  going  on 
when  the  animal  is  fattened,  they  do  not  tell  us  how  the 

O 


2IO 


FOOD  REQUIRED 


[chap. 


food  gets  utilised  nor  what  proportion  of  it  is  stored 
up  within  the  animal.  It  is  obvious  that  the  animal 
by  no  means  adds  to  itself  the  whole  of  its  food,  or  even 
the  whole  of  what  it  digests.  We  have  already  seen 
that  a  considerable  proportion  of  food  is  required  simply 
for  maintenance,  being  burnt  as  fuel  to  keep  up  the 
heat  of  the  body  and  carry  on  the  internal  work  of  the 
organs.  But  even  if  we  make  deductions  of  the  matter 
used  for  maintenance,  we  shall  not  find  that  the  rest  of 
the  digestible  food  is  stored  up  in  the  body  of  the  fatten- 
ing animal.  Lawes  and  Gilbert  put  together  a  large 
number  of  statistics  relating  to  animals  being  fattened 
in  the  ordinary  way  upon  the  farm ;  determining  their 
weight  from  time  to  time,  and  the  weight  and  composi- 
tion of  the  food  which  they  had  been  receiving.  From 
these  results  Table  XXII.  was  constructed,  which  shows 


Table  XXII.— Relation  of  Food  consumed  to  Live 
Weight  Increase. 


Dry  Substance  of  Food 

. 

■»3 

42 

a 

9n- 

Consumed. 

it 

Si 

o  a 

u 

0 

sis 

tl! 

Sag 

1! 

11 

1^1 

Days. 

Lb. 

Lb. 

Lb. 

Lb. 

Lb. 

Oxen      . 

27 

112 

87 

I46i 

121 

I3-0 

9.4 

50 

Sheep     . 

19 

307 

143 

20\ 

159 

9.2 

17.2 

54 

Pigs        . 

33 

104 

5« 

48 

270 

4.8 

56.2 

63 

Dry  matter  of  solid  excrement  and  urine,  exclusive  of  litter. 


that  to  make  i  lb.  of  increase  in  the  live  weight  of 
oxen,  1 3  lb.  of  dry  food  material  was  necessary,  while 
about   9  lb.   were   wanted   in   the   case   of  sheep   and 


XI.]  RELATION  OF  FOOD  TO  INCREASE  211 

5  lb.  in  the  case  of  pigs  to  produce  the  same  increase 
of  I  lb.  in  the  live  weight.  Of  course,  these  results  are 
only  very  approximate,  particularly  as  they  do  not  take 
into  account  the  nature  of  the  food,  but  express  the  kind 
of  results  which  may  be  expected  on  the  very  mixed 
diet  prevailing  in  England.  We  have  already  seen  that 
not  only  do  foods  differ  very  greatly  in  composition,  but 
that  much  more  of  the  digestible  part  of  the  food  is 
available  for  the  service  of  the  animal  in  the  case  of 
concentrated  foods  than  with  coarse  fodders,  which 
require  a  large  expenditure  of  energy  for  their  diges- 
tion. Moreover,  these  figures  by  Lawes  and  Gilbert 
are  average  figures  spread  over  the  whole  period  of 
fattening,  and  if  we  examine  more  closely  the  rate  of 
increase,  we  shall  find  that  a  given  weight  of  food  is 
much  more  effective  in  producing  live  weight  in  the 
earlier  than  in  the  later  stages  of  fattening.  This  was 
well  brought  out  in  some  other  experiments  of  Lawes 
and  Gilbert,  in  which  pigs  were  fattened  for  a  period  of 
ten  weeks.  In  the  first  month,  less  than  4  lb.  of  food 
produced  an  increase  of  i  lb.  in  the  live  weight ;  during 
the  second  month,  5  lb.  of  food  were  required  to  produce 
the  same  increase ;  while  in  the  last  fortnight,  as  much 
as  6  J  lb.  of  food  were  consumed  for  each  pound  of  weight 
put  on.  Thus  a  farmer  who  is  fattening  animals  for 
market  should  always  remember  that  it  is  the  last  few 
pounds  which  are  the  most  expensive  to  produce,  and 
that  he  may  easily  spend  very  much  more  than  he 
obtains  in  getting  up  the  final  finish  of  the  animal. 

The  exact  value  of  the  various  constituents  of 
fattening  stuffs  in  producing  increased  live  weight  has 
been  worked  out,  since  the  experiments  of  Lawes  and 
Gilbert,  by  the  much  more  elaborate  investigations  which 
demand  the  use  of  a  respiration  chamber.  By  these 
means  an  exact  balance-sheet  is  obtained  for  the  period 


212  FOOD  REQUIRED  [chap. 

during  which  the  animal  is  under  investigation,  the 
composition  of  the  food  is  known,  the  composition  of 
the  excreta  is  determined,  and  also  the  amount  of 
carbon  dioxide  and  methane  given  off  by  the  animal. 
The  difference  between  the  carbon  in  the  food  and  the 
excreta  will  represent  the  carbon  stored  up  in  the  body 
as  fat  and  lean  meat,  and  as  the  amount  of  nitrogen 
stored  is  similarly  determined,  we  can  further  calculate 
how  much  of  the  carbon  has  been  stored  in  the  form 
of  fat.  From  this  kind  of  experiment  it  has  been 
determined  that  of  i  lb.  of  pure  digested  fat  about  \  lb. 
is  stored  in  the  body,  supposing  that  the  fat  is  given  in 
addition  to  a  maintenance  ration  and  none  of  it  is 
needed  by  the  animal  for  general  purposes.  The 
proportion  varies  somewhat  with  the  nature  of  the  fat, 
the  range  being  from  about  47  per  cent,  to  about  60 
per  cent,  as  a  maximum.  Of  pure  digested  protein 
about  23  per  cent,  could  be  stored  as  fat.  Digested 
starch  and  crude  fibre  can  give  rise  to  about  25  percent, 
of  their  weight  as  fat,  while  sugar  gives  rise  to  less  than 
19  per  cent.  These  proportions,  however,  refer  only  to 
the  pure  food  constituents  in  their  digested  form.  In 
many  feeding  stuffs  the  digestible  constituents  do  not 
attain  these  full  values,  a  large  and  varying  proportion 
being  spent  in  the  work  of  digestion.  Roughly  speaking, 
in  the  various  cakes  and  meals  the  digestible  constituents 
possess  their  full  value  for  making  fat,  such  as  is  shown 
by  the  pure  constituents  themselves.  It  is  in  the  case 
of  the  coarse  fodders,  such  as  bran,  hay,  and  roots  like 
mangolds,  that  a  deduction  must  be  made  from  the  full 
value  to  express  the  value  to  the  animal  in  producing 
increase.  We  can,  in  fact,  attach  to  each  feeding  stuff 
which  has  been  under  investigation  a  factor  showing  the 
percentage  of  their  full  value  which  the  digestible 
constituents  will  possess  in  that  particular  feeding  stuff. 


XI.]  VALUATION  OF  FEEDING  STUFFS  213 

For  example,  the  factor  for  meadow  hay  is  about  70  per 
cent.,  which  means  that  the  digestible  protein  in  the 
meadow  hay,  instead  of  being  capable  of  yielding  23  per 
cent,  of  its  weight  of  fat  stored  up  in  the  animal  (suppos- 
ing the  hay  is  added  to  the  diet  required  for  maintenance 
so  that  it  can  all  be  utilised  for  making  increase),  will 
only  yield  70  per  cent,  of  its  full  value  of  23  per  cent,  i.e. 
the  digestible  protein  of  the  meadow  hay  will  only  be 
converted  into  16  per  cent,  of  its  weight  of  stored  up 
fat.  In  the  calculations  in  the  preceding  chapter 
(p-  193)  we  deducted  3  per  cent,  of  the  energy  con- 
tained in  the  digested  part  of  cotton  cake,  but  40  per 
cent,  of  the  energy  contained  in  digested  meadow  hay, 
in  order  to  find  the  dynamic  energy  available  for  work 
or  increase. 

We  can  make  use  of  the  values  which  have  been 
deduced  for  the  digestible  constituents  in  the  feeding 
stuffs  so  as  to  obtain  a  single  figure  which  sums  up  the 
relative  value  of  the  feeding  stuff.  We  have  already 
attempted  to  do  this  in  a  somewhat  imperfect  fashion 
when  discussing  the  analysis  of  the  feeding  stuffs.  We 
have  taken  as  a  kind  of  guide  the  fact  that  the  fats  are 
about  two  and  a  quarter  times  as  valuable  for  fuel 
purposes  as  the  carbohydrates,  and  have  also  made  the 
assumption  that  the  proteins  are  worth  about  as  much 
as  fats  (a  commercial  assumption  which  takes  into 
account  the  value  of  its  nitrogen  as  well  as  the  fuel 
value  of  the  protein).  Thus,  we  can  add  to  the  per- 
centages of  carbohydrates  the  percentages  of  fat  and 
protein  multiplied  by  2-3,  and  so  obtain  the  number  of 
units  which  represent  the  value  of  the  food  in  terms  of 
carbohydrates.  Such  a  valuation,  which  is  all  or  more 
than  the  market  takes  account  of,  possesses  little  exact 
value,  because  it  neglects  the  digestibility  of  the  various 
constituents.     But  if  we  base  our  calculations  upon  the 


214  FOOD  REQUIRED  [chap. 

digestible  portions  only  of  the  food,  the  results  are  still 
vitiated  by  failing  to  take  into  account  the  work  that  is 
spent  in  the  digestion  of  the  food.  Such  a  unit  system 
is  not  greatly  in  error  when  we  can  confine  it  to  com- 
parisons between  the  highly  concentrated  foods  that 
possess  nearly  full  value  (p.  183),  but  breaks  down  when 
the  comparison  is  made  between  a  concentrated  and  a 
comparatively  low-grade  fodder. 

For  example,  working  from  Table  XVII.  the  crude 
analysis  shows  for  decorticated  cotton  cake  41  per  cent, 
of  protein  and  9  of  oil,  50  in  all,  which,  multiplied  by  2-3, 
give  115.  Add  26  per  cent,  of  carbohydrates,  and  the 
total  food  units  amount  to  141.  Meadow  hay,  with  10 
of  protein  and  2-5  of  fat,  to  be  multiplied  by  2-3,  give  .28 
units,  which,  added  to  42  of  carbohydrates,  give  70  food 
units ;  making  the  hay  just  about  half  as  valuable  as  the 
cotton  cake.  If  we  consider  the  digestible  constituents 
only,  the  cotton  cake  shows  34  per  cent,  of  protein  and 
8-5  of  oil,  which,  multiplied  by  2-3,  give  98,  to  which  must 
be  added  20  of  carbohydrates  and  fibre,  making  a  total 
of  118  digestible  food  units.  Meadow  hay  contains  4 
of  digestible  proteins  and  i  of  fat,  4-I- 1  X  2-3  =  1 1  ;  adding 
41  digestible  carbohydrates  and  fibre  we  get  52  total  food 
units,  considerably  less  than  half  the  figure  for  cotton 
cake.  The  soundest  method  of  comparison  which  we 
have  now  to  explain  would  give  figures  of  7 1  and  3 1 
respectively,  bringing  the  meadow  hay  still  further 
below  the  cotton  cake. 

The  basis  of  comparison  is  derived  from  considera- 
tions— (i)  of  the  digestible  constituents  of  the  food;  (2) 
of  the  facts  we  have  just  stated,  that  a  pound  of  fat  in 
the  food  will  be  stored  to  the  extent  of  about  50  per 
cent,  whereas  a  pound  of  protein  only  gives  rise  to  less 
than  a  quarter  of  a  pound  of  fat,  and  a  pound  of  starch 
to  about   the   same ;   and  (3)  of  the  value  that  these 


XI.]  STARCH  EQUIVALENTS  215 

constituents  possess  in  each  food  when  deduction  has 
been  made  for  the  work  spent  in  digestion.  In  order  to 
arrive  at  a  convenient  number  for  our  comparisons,  we 
shall  take  starch  as  a  basis  and  reduce  the  digestible 
constituents  of  the  other  foods  into  terms  of  their 
equivalents  of  starch.  For  example,  we  have  just  seen 
that  a  pound  of  digested  fat  in  the  food  will  give  rise  to 
half  a  pound  of  fat  in  the  increase,  whereas  a  pound  of 
starch  only  gives  rise  to  a  quarter  pound  of  fat  in  the 
increase  ;  so  that  if  we  call  100  the  value  of  starch,  then 
we  should  have  to  express  the  value  of  fat  by  200. 
Similarly,  pure  protein  has  a  fat-making  value  about  90 
per  cent,  of  that  of  starch,  so  that  the  equivalent  in 
starch  of  100  of  protein  would  be  90.  In  this  case  we 
are  dealing  with  pure  food  constituents,  but  exactly  the 
same  principle  may  be  followed  for  each  feeding  stuff  as 
a  whole.  We  can  obtain  a  single  number  which  repre- 
sents the  number  of  pounds  of  starch  which  would  have 
the  same  fattening  effect  as  100  lb.  of  the  food 
in  question.  This  starch  equivalent  represents  the 
summing  up  of  the  value  of  the  fat,  protein,  carbo- 
hydrates, and  fibre  respectively  in  the  food,  after  the 
deduction  brought  about  by  the  work  spent  in  digestion. 
Table  XVII.,  with  its  list  of  food  compositions  and 
digestible  constituents,  also  gives  the  starch  equivalents, 
which  range  from  as  high  as  120  for  linseed  to  as  low  as 
1 1  for  wheat  straw.  Such  a  table  of  starch  equivalents 
forms  the  only  sound  basis  for  the  comparison  of  the 
value  of  feeding  stuffs,  and  though  there  are  many  other 
factors  to  be  taken  into  account  in  making  up  rations 
for  farm  animals,  these  figures  should  be  considered  in 
deciding  which  are  the  cheaper  of  the  concentrated 
feeding  stuffs  which  can  be  purchased,  and  also  in  what 
quantities  they  should  be  used  to  replace  one  another 
in  a  ration.     Of  course  they  refer  only  to  the  fattening 


2i6  FOOD  REQUIRED  [chap. 

increase,  and  they  are  based  upon  experiments  with 
ruminant  animals — sheep  and  oxen.  They  do  not  hold 
exactly  for  fattening  pigs,  nor  do  they  refer  to  the 
production  of  energy  in  working  horses.  However,  the 
energy  which  becomes  available  for  work  purposes  is 
derived  from  the  digestible  constituents  after  deduction 
of  the  work  spent  in  digestion  in  just  the  same  way  as 
the  materials  available  for  increase,  so  that  no  consider- 
able error  is  introduced  if  we  take  the  starch  equivalent 
of  a  food  as  representing  the  energy  that  would  be 
available  for  work  purposes  as  well  as  the  surplus  that 
will  be  available  for  making  increase  of  live  weight. 

One  other  point,  however,  we  must  bear  in  mind.  The 
starch  equivalent  takes  into  account  only  the  fat-making 
power  of  the  food,  and  pays  no  attention  to  the  nitrogen 
it  contains  nor  the  requirements  of  the  animal  for 
nitrogen.  Suppose  we  have  arranged  a  certain  ration 
that  will  supply  the  requirements  of  the  animal  as 
regards  energy  or  fat  production,  basing  the  ration  upon 
the  starch  equivalent  of  the  foods ;  we  have  then  also  to 
make  a  second  calculation  of  the  albuminoid  ratio  of  the 
ration  in  order  to  make  sure  that  the  animal  is  getting 
the  proper  amount  of  nitrogen.  But  with  all  the  know- 
ledge that  has  been  derived  from  the  experiments  upon 
feeding  animals,  knowledge  which  is  summed  up  in  the 
starch  equivalents,  it  would  be  extremely  unwise  to 
begin  to  construct  rations  for  farm  animals  on  a  priori 
principles,  considering  that  they  require  a  certain  number 
of  starch  equivalents  per  diem  and  a  particular 
albuminoid  ratio,  and  assuming  that  it  is  a  matter  of 
indifference  how  these  units  are  arrived  at.  The  better 
plan  is  to  take  as  our  starting-point  certain  well- 
recognised  rations  which  have  been  justified  in  practice 
for  the  particular  purpose  in  view,  and  see  how  they  can 
be    modified    to    secure   equal   efficiency   but    greater 


XI.]  FOOD  RATIONS  i\1 

cheapness.  Sometimes  also  we  find  that  animals  are 
being  grossly  overfed,  and  this  we  can  detect  by  finding 
that  the  total  number  of  starch  equivalents  fed  is  greatly 
in  excess  of  the  number  in  a  standard  ration. 

This  system  of  starch  equivalents  to  represent  the 
relative  value  of  the  different  feeding  stuffs  is  really  the 
return  to  one  of  the  earliest  methods  by  which  it  was 
proposed  to  bring  these  substances  into  comparison. 
Early  in  the  nineteenth  century  Thaer,  one  of  the  first 
of  the  German  agriculturists  to  apply  science  to  the 
feeding  of  animals,  attempted  to  draw  up  a  table  of 
what  he  called  "  hay  values  "  for  the  different  foods  then 
available,  these  hay  values  being  the  equivalents  in 
good  hay  of  ICXD  lb.  of  the  food  in  question.  Thaer's 
hay  values,  however,  were  almost  entirely  based  upon 
the  amount  of  nitrogen  contained  in  food,  and  it  is 
rather  characteristic  of  the  change  that  has  passed  over 
the  science  of  feeding  stuffs  to  find  that  the  new  starch 
equivalents  upon  which  we  now  base  our  comparisons  of 
foods  take  no  account  of  the  nitrogen  the  food  contains. 

A  few  examples  may  now  be  given  of  the  use  of  the 
starch  equivalents  in  compounding  rations. 

The  following  ration  was  given  to  heavy  dray  horses 
working  long  journeys  : — 


Lb. 

Oats      .... 

Peas      .... 

Beans    .... 

Bran      .... 

Clover  and  Rye  Grass  Hay     . 

17 

and  it  became  desirable  to  replace  the  peas,  which  were 
no  longer  obtainable  cheaply.  From  Table  XVII.  we 
learn  that  peas  possess  a  starch  equivalent  of  70  and  an 
albuminoid  ratio  of  i  :  3J.  Now  maize  possesses  a 
starch  equivalent  of  68,  so  that  they  could  be  substituted 


2l8 


FOOD  REQUIRED 


[chap. 


for  peas  without  any  serious  loss  in  the  supply  of  energy  ; 
to  be  exact,  5  lb.  2\  oz.  of  maize  would  be  equivalent  to 
5  lb.  of  peas.  But  the  albuminoid  ratio  of  maize  is 
much  lower  than  that  of  peas,  so  it  is  necessary  to  know 
if  the  albuminoid  ratio  of  the  new  ration  is  below  what 
might  be  considered  safe.  This  may  be  calculated  as 
follows,  still  using  Table  XVII. : — 

Digestible  Constituents. 


Food. 

Weight. 

Protein. 

Oil. 

carbohydrates. 

Food 
Units 
(X2-3). 

Per 

Food 

Per 

Per 

Food 

Cent. 

Units. 

Cent. 

Cent. 

Units. 

Lb. 

•- 

Oats     . 

5 

9 

45 

5-2 

60 

45 

225 

Maize  . 

5 

7 

35 

4-5 

52 

68 

340 

Beans  . 

5 

19 

95 

1-2 

14 

48 

240 

Bran     . 

4 

10 

40 

3 

28 

45 

180 

Meadow  Hay 

^'% 

4 

34 

I 

20 

41 

348 

Clover  Hay . 

8.5 

5*5 

47 

1-5 

29 

35 

323 

296 

203 

1656 

1859 

By  multiplying  the  percentages  of  digestible  protein  by 
the  weight  of  each  food  we  obtain  in  the  third  column  a 
total  of  296  digestible  protein  food  units  in  the  ration. 
To  obtain  the  number  of  digestible  non-protein  units  we 
must  similarly  multiply  their  percentages  by  the  weight 
of  food,  but  the  oil  percentages  must  be  further  multi- 
plied by  2-3  to  make  them  equivalent  to  the  carbo- 
hydrates. Thus  we  get  a  total  of  203  food  units  from 
the  oil  in  the  ration  and  1656  from  the  carbohydrates, 
making  1859  non-protein  food  units  in  all.  This  total 
must  be  divided  by  296,  the  number  of  protein  units,  in 
order  to  obtain  the  albuminoid  ratio  of  the  whole.  As 
in  the  case  in  question,  this  is  still  as  narrow  as  i  :  6,  we 


XI.]  FOOD  RATIONS  219 

may  assume  that  the  ration  is  well  above  the  safety 
limit,  and  that  the  maize  can  be  substituted  for  the 
peas  without  affecting  the  horses.  Instead  of  calculat- 
ing out  the  albuminoid  ratio  of  the  whole,  we  might 
have  found  how  much  digestible  protein  the  new  ration 
contained,  as  follows : — 


Lb. 

Oats,  5  lb.  at  9  per  cent. 

= 

•45 

Maize,  5  lb.  at  7      „ 

= 

•35 

Beans,  5  lb.  at  19    „ 

= 

•95 

Bran,  4  lb.  at  10      „ 

.     = 

•40 

Meadow  Hay,  8i  lb.  at  4  per  cent. 

= 

•34 

Clover  Hay,  8J  lb.  at  si        „ 

.     = 

•47 

2-96 


Thus  the  horse  would  be  getting  nearly  3  lb.  of  digestible 
protein  per  diem,  and  Table  XXIII.  shows  that  a  horse 
in  heavy  work  requires  2  lb.  per  diem  per  1000  lb. 
live  weight.  Thus,  3  lb.  would  be  sufficient  if  the  horses 
in  question  did  not  weigh  more  than  1500  lb.  To  take 
another  case,  a  fattening  ration  for  cattle  contained  : — 

Lb. 

Swedes      .  .  .  .84 

Hay  ....        12 

Linseed  Cake        ...  5 

and  it  was  desired  to  substitute  cotton  cake  and  barley 
meal  for  the  linseed  cake.  Five  pounds  of  linseed  cake 
is  equivalent  to  5x76-^100=3-8  lb.  starch;  it  also 
contains  5x25-^100=1-25  lb.  protein.  Barley  has  a 
starch  equivalent  of  74  and  9  per  cent,  of  digestible 
protein,  so  that  2  J  lb.  of  barley  meal  would  be  equivalent 
to  2jx-74=  1-65  lb.  starch,  and  would  contain  -22  lb.  of 
digestible  protein.  Three  pounds  of  decorticated  cotton 
cake  with  a  starch  equivalent  of  71  and  34  per  cent,  of 
digestible   protein   would   have   a  starch  equivalent  of 


220 


FOOD  REQUIRED 


[chap. 


2-13  lb.,  and  would  contain  i-02  lb.  protein.  Thus  the 
two  would  together  have  a  starch  equivalent  of 
1-85  4- 2- 13  =  3-98,  and  would  contain  •22  +  1-02=  1-24  lb. 
protein,  or  the  fattening  value  of  the  ration  has  been 
increased  a  trifle,  while  the  amount  of  protein  remains 
the  same,  so  that  probably  \  lb.  could  be  taken  off  the 
barley  meal,  as  the  amount  of  protein  is  already  pretty 
high. 

Dairy  cows  were  receiving : — 


Lb. 

Beans 

4 

Oats 

3 

Linseed  Cake 

2 

Bran 

4 

Straw  Chaff 

5 

Hay 

6 

Straw 

6 

and  it  was  desirable  to  substitute  gluten  meal  and  maize 
as  much  as  possible  for  the  other  concentrated  foods, 
which  had  become  dear.  We  may  calculate  as 
follows : — 


Starch  Equivalent. 

Protein. 

Beans,  4  lb.  . 
Oats,  3  lb.     . 
Linseed  Cake,  2  lb. 

Per  cent. 
67 
63 
76 

Lb. 
2-68 
1-89 
1-52 

Per  cent. 

19 

9 

25 

Lb. 

•76 
•27 

•50 

Total      . 

... 

6-09 

... 

i^53 

Thus  we  have  to  make  up  6- 1  lb.  of  starch  equivalent 
and  1-5  lb.  of  protein.  Gluten  meal  has  a  starch 
equivalent  of  "JJ  and  33  per  cent,  of  digestible  protein  ; 
maize  has  a  starch  equivalent  of  84,  but  only  7  per  cent- 
of  digestible  protein ;   thus  rather  a  smaller  weight  of 


XL] 


FOOD  RATIONS 


221 


these  foods  will  serve,  but  the  gluten  meal  must  pre- 
dominate in  order  to  keep  up  the  protein.  We  may 
try:- 


Starch  Equivalent. 

Protein. 

4  lb.  Gluten  Feed 

gives 
4  lb.  Maize  gives  . 

Per  cent. 

77 
84 

Lb. 

308 
3-36 

Per  cent. 

33 
7 

Lb. 

1-32 
0-28 

Total     . 

... 

6.44 

... 

I -60 

and  obtain  rather  more  both  of  starch  equivalent  and  of 
protein.  It  would  now  be  wise  to  replace  2  lb.  of  the 
gluten  feed  by  decorticated  cotton  cake,  both  to  give 
variety  and  impart  consistency  to  the  butter  fat. 

We  then  have — 


Starch  Equivalent. 

Protein. 

2  lb.  Decorticated 
Cotton  Cake      . 
2  lb.  Gluten  Feed . 
4  lb.  Maize   . 

Per  cent. 

71 
77 
84 

Lb. 

1.42 
1.54 
3-36 

Per  cent. 

34 

33 

7 

1 
Lb. 

0-68 
0.66 

0-28 

Total     . 

... 

6.32 

... 

1-62 

so  that  we  could  reduce  the  amount  of  the  mixture  by 
about  \  lb.  per  head  in  order  to  obtain  an  almost  exact 
equivalent  to  the  original  ration  both  in  the  supply  of 
energy  and  of  protein. 

One  more  example  will  suffice  to  illustrate  the 
method ;  fatting  pigs  per  100  lb.  live  weight 
received : — 

Lb. 

Barley  Meal  ...  4 

Pea  Meal  .  .  .  ,  i 


222  FOOD  REQUIRED  [chap. 

which  have  to  be  replaced  by  maize  and  decorticated 
cotton  cake.  Four  pounds  barley  meal  will  supply  2-96  lb. 
starch  equivalent  and  0-36  lb.  digestible  protein ;  i  lb. 
pea  meal  will  add  070  lb.  starch  equivalent  and  0-17  lb. 
protein  :  total,  37  lb.  starch  and  0-53  lb.  protein.  Four 
pounds  maize  will  supply  3-36  starch  and  0-28  protein, 
to  which  \  lb.  of  cotton  cake  will  add  0-35  of  starch  and 
0-17  protein,  making  a  total  of  3-71  lb.  starch 
equivalent  and  0-45  digestible  protein.  The  protein  is 
a  little  lower  in  this  ration  than  in  that  which  it  was 
intended  to  replace,  but  the  albuminoid  ratio  is  still 
narrow  enough  for  pigs  that  have  made  a  good  deal  of 
growth  and  are  chiefly  putting  on  fat.  Another  ounce 
or  two  of  cotton  cake  and  a  little  less  maize  would  make 
the  ration  almost  exactly  equivalent  to  the  old  one. 

Such  are  the  uses  to  which  the  following  tables  can 
be  put  by  a  practical  farmer.  In  Table  XXII  I.  are  given 
certain  standard  rations  which  have  been  worked  out  by 
Kellner  from  the  results  of  a  very  large  number  of 
exact  experiments  ;  from  these  he  may  learn  the  weight 
of  starch  equivalent  and  protein  that  is  appropriate  to 
various  farm  animals.  By  the  aid  of  Table  XVII.  the 
farmer  can  calculate  the  starch  equivalents  and  digestible 
protein  in  the  rations  he  is  using,  and  correct  them  if 
they  depart  widely  from  the  quantities  given  by 
Kellner.  Finally,  if,  to  suit  the  fluctuations  of  the 
market  or  the  materials  available  on  the  farm,  he 
wishes  to  modify  his  current  ration,  he  can  from  this 
same  table  calculate  the  quantities  of  other  foods 
necessary  to  replace  those  to  which  he  has  hitherto 
been  accustomed. 


XI.] 


STANDARD  RATIONS 


223 


Table  XXIII.— Standard  Rations  (Kellner), 
per  1000  lb.  live  weight  per  day. 


Dry  Matter  in 
Total  Ration. 

Digestible. 

Animal. 

Protein. 

starch 
Equivalent. 

Horse,  light  work    . 
Horse,  medium  work 
Horse,  heavy  work  • 

18—23 
21—26 

23—28 

10 
1.4 
2-0 

9.2 
II.6 
15-0 

Fattening  Cattle- 
At  550  lb.  live  weight 
At  770  lb.  live  weight 
At  950  lb.  live  weight 

26 
26 
26 

2-8 
2.2 
1-5 

14.4 
11.2 
100 

Milch  Cattle- 
Yielding    10    lb.    milk    per 

1000  lb.  live  weight  . 
Yielding  20  lb.  milk      • 
Yielding  30  lb.  milk     . 
Yielding  40  lb.  milk     . 

22-27 
25—29 
27—33 
27—34 

1.0 — 1.3 
1.6— 1.9 

2.2—2-5 

2.8—3-2 

7-8-8-3 
9.8— 1 1 -2 

11-8-13-9 
13-9- 16-6 

Fattening  Lambs — 
65  lb.  live  weight 
no  lb.  live  weight 
Full  grown  .... 

31 
28 
24-32 

3-5 
2-5 
1.6 

17 
15 
14-5 

Fattening  Pigs— 
44  lb.  live  weight 
no  lb.  live  weight 
200  lb.  live  weight 

28 

6.2 
4-5 
3-0 

33-8 

32 

24-5 

CHAPTER   XII 

FARMYARD   MANURE 

Composition  of  Animal  Excretions.  Litter.  Changes  taking  place 
during  the  Making  and  Storage  of  Manure.  Losses  of 
Nitrogen  in  Manure-making  —  Unavoidable  or  due  to 
wasteful  Methods.  Composition  of  Farmyard  Manure  from 
various  Sources.  Care  of  Farmyard  Manure.  Farmyard 
Manure  as  a  Fertiliser.  Value  of  Farmyard  Manure.  Valua- 
tion of  Manure  Residues  derived  from  the  Consumption  of 
Purchased  Feeding  Stuffs.     Cost  of  Farmyard  Manure. 

In  the  preceding  chapter  we  have  learned  that  the  food 
of  animals  contains  various  substances  which  are  also 
food  for  plants.  The  fat,  the  fibre,  and  the  carbo- 
hydrates in  a  feeding  stuff  are  useless,  because  being 
only  compounds  of  carbon,  hydrogen,  and  oxygen,  they 
are  as  far  as  they  are  digested  resolved  into  carbon 
dioxide  and  water,  and  even  their  indigestible  portions 
when  they  reach  the  soil  cannot  feed  the  plant.  The 
nitrogen,  however,  that  the  feeding  stuff  contains  is  of 
the  first  importance  to  the  plant,  and  the  phosphoric 
acid  and  the  potash  which  are  also  present  in  the  ash 
are  equally  indispensable  elements  of  the  plant  food. 
We  have  further  learnt  that  the  animal  only  retains  in 
its  body  a  comparatively  small  proportion  of  the 
nitrogen  and  other  valuable  constituents  of  the  food. 
The  actual  proportion  retained  depends  upon  the  age 
of  the  animal ;  a  young  animal  putting  on  flesh,  or  a 
cow  in  full  milk,  take  from  the  food  more  of  the  nitrogen 

224 


XII.]  ORIGIN  OF  MANURES  225 

and  the  phosphoric  acid  than  animals  which  are  stationary 
in  weight,  while  animals  in  the  last  stages  of  fattening 
hardly  retain  anything  at  all.  It  is  because  the  animal 
thus  keeps  back  so  little  of  the  plant  food  which  was,  in 
the  first  place,  taken  from  the  soil  for  the  production 
of  the  vegetable  feeding  stuff,  that  the  excrements  of 
animals  has  always  been  regarded  as  the  most  valuable 
of  fertilisers  since  any  settled  agriculture  began.  In 
fact,  it  is  only  comparatively  recently  that  any  other 
fertiliser  has  been  known,  for  though  various  industrial 
residues,  such  as  woollen  rags,  clippings  of  hoofs  and 
horns,  bones,  and  malt  dust  have  been  utilised  as 
manures  for  the  last  two  or  three  hundred  years,  their 
efficacy  was  very  limited,  and  the  business  of  crop- 
raising  centred  round  the  proper  use  of  farmyard 
manure.  The  great  range  of  artificial  manures  now 
available,  which  are  either  derived  from  industrial  pro- 
cesses or  represent  the  accumulated  fertility  of  some 
other  country,  have  all  come  into  use  since  about  1840. 

Farmyard  manure  consists  essentially  of  the  excreta 
of  the  various  animals,  horses,  cattle,  and  pigs  kept  in 
the  farmyards,  mixed  with  the  litter — straw  or  other- 
wise— which  is  used  to  absorb  the  urine  and  keep  the 
animals  clean.  Very  slight  consideration  shows  us, 
however,  that  in  the  farmyard  manure  that  goes  on  the 
land  we  are  dealing  with  a  very  different  product  from 
the  fresh  mixture  of  straw  and  excreta.  The  mixture, 
in  fact,  undergoes  great  changes  during  the  time  it  is 
under  the  feet  of  the  animals,  and  these  changes  are 
mainly  brought  about  by  bacteria.  The  first  change 
taking  place  while  the  manure  is  being  "  made,"  as  a 
farmer  would  say,  is  continued  to  a  greater  or  less 
extent  when  the  manure  is  afterwards  removed  from  the 
yards  or  the  boxes  and  made  up  into  heaps.  At  first 
the  straw  of  the   manure  shows   little   alteration,  but 

P 


226  FARMYARD  MANURE  [chap. 

as  the  making  process  proceeds  it  is  partly  broken  down 
by  the  hoofs  of  the  animals  and  partly  by  bacterial 
decay,  which  latter  change  proceeds  still  further  during 
the  storage  process  until  no  trace  of  the  straw  structure 
may  remain,  but  the  whole  material  has  passed  into  a 
uniform  brown  or  black  mass.  The  farmer  is  accustomed 
to  speak  of  the  fresh  straw  in  manure  as  "  long,"  while 
the  old  fermented  material  he  calls  "  short." 

Before  considering  in  detail  the  changes  which  go  on 
during  the  processes  of  making  and  storage,  we  must 
refer  back  to  what  we  have  already  learnt  concerning 
the  fate  of  the  valuable  materials  in  the  feeding  stuffs, 
fixing  our  attention  for  the  time  being  upon  the 
nitrogen.  It  will  be  remembered  that  portions  of  the 
nitrogenous  compounds  in  the  food  are  indigestible,  and 
are  excreted  by  the  animal  in  the  faeces ;  that  part, 
however,  which  is  digested  is  excreted  in  the  form  of 
urea  in  the  urine,  except  for  a  small  portion  which  the 
animal  may  retain  in  the  body.  This  division  of  the 
nitrogen  in  the  food  represents  a  great  difference  in  its 
value  as  a  fertiliser.  The  nitrogen  compounds  in  the 
faeces,  since  they  have  resisted  the  attack  of  the  digestion 
processes,  will  be  very  slowly  attacked  in  the  soil  by 
the  bacteria  which  have  to  break  down  and  convert 
them  into  ammonia  and  nitrates  before  they  can  feed 
the  plant.  Such  materials,  then,  are  slow  in  their  action 
as  fertilisers,  and  remain  for  a  very  long  time  unchanged 
in  the  soil.  The  nitrogen,  however,  in  the  liquid  portion 
of  the  manure  in  which  it  is  present  in  the  shape  of 
urea,  will  change  very  rapidly  into  ammonia,  so  that  it 
is  an  extremely  active  fertiliser,  and  far  more  valuable 
than  the  solid  parts  of  the  manure  which  would  contain 
the  same  amount  of  nitrogen.  The  same  considerations 
apply  equally  to  the  phosphoric  acid  and  potash ; 
whatever   part   of  these   constituents    in    the    food    is 


XII.]  MANURIAL  VALUE  OF  EXCRETA  227 

digested  gets  excreted  in  a  liquid  and  soluble  form,  and 
is  therefore  in  an  available  state  for  the  plant,  whereas 
the  same  constituents  in  the  faeces  are  insoluble,  and 
only  reach  the  plant  after  the  lapse  of  some  considerable 
time.  We  have  also  learnt  in  the  previous  chapter  that 
the  richer  and  more  concentrated  a  food  is,  the  greater 
is  the  proportion  of  its  nitrogen  that  is  digested.  For 
example,  decorticated  cotton  cake  contains  nearly  7  per 
cent,  of  nitrogen,  and  about  nine-tenths  of  that  nitrogen 
is  digested  and  reappears  in  the  soluble  active  form  of 
urea;  whereas  hay  only  contains  about  i\  per  cent,  of 
nitrogen,  of  which  barely  half  is  digestible,  while  the 
other  half  is  excreted  in  a  solid  form  and  will  form  but 
a  slow-acting  fertiliser.  Thus  an  animal  which  is  fed 
with  concentrated  cakes  and  meals,  such  as  a  bullock 
during  the  fattening  process,  will  be  giving  rise  to  much 
richer  manure  than  animals  which  are  being  kept  in  a 
store  condition  and  only  receiving  such  low-grade  foods 
as  hay,  straw,  and  roots,  even  though  the  same  amount 
of  nitrogen  is  being  consumed  in  the  two  cases.  We 
have  thus  a  number  of  factors  affecting  the  composition 
of  farmyard  manure.  Young  growing  stock  take 
nitrogen  from  the  food  in  order  to  make  flesh,  and 
phosphoric  acid  in  order  to  make  bone  ;  milch  cows  take 
both  phosphoric  acid  and  nitrogen  for  their  milk ;  store 
stock  and  working  horses  do  not  receive  very  concen- 
trated food,  and  in  their  turn  give  rise  to  comparatively 
poor  manure.  The  animals  themselves  induce  a  certain 
amount  of  difference,  and  though  the  composition  of  the 
excreta  varies  so  much  with  the  age  and  food  that 
analyses  are  not  of  much  information  unless  large 
numbers  are  given,  it  will  be  found  that  the  urine  of 
sheep  and  horses  is  more  concentrated  than  that  of 
cattle  and  pigs,  and  similarly,  that  the  solid  excreta 
of  the  two  former  are  also  drier.     It  is  this  greater 


228  FARMYARD  MANURE  [chap. 

dryness  and  richness  which  causes  the  gardener  to 
describe  horse  manure  as  "  hotter "  than  that  produced 
by  cattle  and  pigs.  Bacterial  changes  take  place  in  it 
more  rapidly,  and  both  a  greater  amount  of  ammonia 
and  a  greater  rise  of  temperature  are  produced  by  the 
fermentation.  The  composition  of  the  resulting  farm- 
yard manure  also  varies,  to  a  certain  extent,  with  the 
litter  employed.  Straw  is,  of  course,  most  general,  and 
though  there  are  differences  in  composition  between 
wheat,  oat,  and  barley  straw,  these  differences  are  not 
great,  and  are  indeed  less  than  the  variation  in  the 
composition  of  any  one  of  them  in  different  seasons. 
Speaking  generally,  the  straw  is  richer  in  cool  seasons 
and  in  more  northern  climates.  The  only  other  sub- 
stance at  all  widely  used  for  litter  is  peat  moss;  it  is 
both  somewhat  richer  in  itself  in  nitrogen  than  is  straw, 
and  possesses  a  greater  absorbing  capacity  for  the 
liquid  portions  of  the  manure.  It  is  doubtful,  however, 
whether  the  extra  nitrogen  in  the  peat  moss  is  of  much 
service  to  the  plant,  nor  do  the  bacterial  changes  in 
making  the  manure  go  on  so  readily  as  with  straw. 

The  changes  we  shall  now  discuss  refer  to  ordinary 
farmyard  manure  made  with  straw,  and  these  changes 
may  be  divided  into  two  groups — those  taking  place  in 
the  carbon  and  the  nitrogen  compounds  respectively. 
So  far  as  the  carbon  compounds — the  fibre  and  other 
carbohydrates  in  straw — are  concerned,  the  chief  change 
that  takes  place  is  the  anaerobic  fermentation  which  we 
have  discussed  when  dealing  with  soils.  A  number  of 
organisms  are  present  in  the  air  and  in  dust,  which  at 
once  attack  the  carbohydrate  material  of  the  straw  and 
begin  to  burn  it  up,  with  the  production  of  carbon 
dioxide.  The  organisms,  however,  soon  use  up  all  the 
oxygen  that  is  contained  in  the  air  entangled  in  the 
manure,  whereupon    the    work    is    taken    up    by   the 


XII.]        LOSS  OF  WEIGHT  IN  DUNG-MAKING  229 

anaerobic  organisms,  which  give  rise  on  the  one  hand 
to  carbon  dioxide  and  marsh-gas  or  hydrogen,  and  on 
the  other  hand  to  the  dark  brown  substance  of  indefinite 
composition  which  we  call  humus.  Analysis  of  the  gas 
derived  from  a  newly  made  dunghill  showed  that  at 
first  a  good  deal  of  hydrogen  was  being  produced,  but 
when  the  dunghill  was  kept  tight  and  moist  the  most 
characteristic  fermentation  was  that  giving  rise  to 
carbon  dioxide  and  marsh-gas.  Only  when  the  mass 
was  allowed  to  get  dry,  so  that  air  could  enter,  did  the 
aerobic  fermentation,  giving  rise  to  carbon  dioxide 
alone,  begin  to  take  place.  It  will  be  seen,  however, 
that  a  considerable  loss  of  dry  material  must  take  place, 
because  whether  the  fermentation  takes  place  in  the 
presence  or  absence  of  air,  solid  carbohydrates  are  being 
converted  into  gases  like  carbon  dioxide  and  marsh-gas. 
We  find,  as  a  matter  of  fact,  that  during  the  making  of 
dung,  something  like  a  quarter  of  the  original  dry  matter 
is  burnt  up,  and  that  by  the  time  the  dung  has  been 
made  and  stored,  this  loss  of  dry  matter  has  been 
increased  to  one-half,  and  may  easily  become  greater 
with  very  old  short  manure. 

Turning  now  to  the  nitrogenous  compounds  which 
form  the  chief  fertilising  elements  of  the  manure,  we 
have  already  said  that  the  most  important  is  the  soluble 
urea  contained  in  the  urine.  This  substance  is  at  once 
attacked  by  bacteria  which  are  always  present  in  cattle 
stalls,  stables,  etc.,  and  by  a  very  slight  chemical  change 
is  converted  into  carbonate  of  ammonia.  Carbonate  of 
ammonia  is  a  substance  which  spontaneously  splits  up 
into  the  two  gases  carbon  dioxide  and  ammonia,  so  that 
when  any  liquid  containing  carbonate  of  ammonia  is 
exposed  to  the  air  and  dried  up  at  all,  the  valuable 
ammonia  is  at  once  converted  into  gas,  and  escapes.  In 
this  way  a  great  loss  of  fertilising  material  can  easily 


230  FARMYARD  MANURE  [chap. 

arise,  and  we  find  as  a  matter  of  practice  that,  despite 
the  utmost  care  that  can  be  taken  in  making  dung,  there 
will  always  be  a  loss  of  nitrogen  due  to  the  volatilisation 
of  the  ammonia  derived  from  the  fermentation  of  the 
urea.  This  loss,  furthermore,  falls  upon  the  most 
valuable  of  the  nitrogen  compounds,  i.e.  upon  those 
which  are  soluble  in  water  and  readily  available  as  food 
for  plants.  A  large  number  of  experiments  have  been 
made  in  which  the  amount  of  nitrogen  supplied  to  the 
animals  was  carefully  determined  and  compared  with 
the  amount  which  was  afterwards  found  in  the  dung 
that  had  been  made,  and  under  the  most  favourable 
conditions  of  practice  the  loss  amounted  to  about  15  per 
cent,  during  the  making  of  the  manure.  The  best 
conditions  are  found  to  be  attained  by  keeping  the 
straw  and  manure  tightly  trodden  down  beneath  the 
feet  of  the  animals,  and  in  a  fairly  moist  condition  so  as 
to  exclude  the  action  of  air.  If  the  straw  was  allowed 
to  remain  in  the  yard  or  box  in  a  loose,  open  condition, 
or  if  the  litter  was  cleared  from  under  the  animals  and 
simply  thrown  into  a  heap  day  by  day,  then  the  losses 
of  ammonia  were  very  much  increased,  and  often  rose  to 
half  of  the  nitrogen  that  had  been  given  in  the  food. 
All  disturbances  of  the  manure  should  be  avoided, 
because  they  result  in  very  active  fermentation,  with  a 
corresponding  increase  in  the  evaporation  of  ammonia. 
For  example,  when  fresh  strawy  manure  is  repeatedly 
turned,  it  is  well  known  that  the  temperature  of  the 
whole  mass  rises  to  70°  or  80°  through  the  fermentation 
that  sets  in,  and  quantities  of  ammonia  are  given  off 
during  the  turning,  as  may  be  detected  by  the  smell. 
The  heat  that  is  generated  by  the  active  bacterial 
fermentation  is  utilised  by  gardeners  in  the  preparation 
of  a  hotbed,  and  it  is  well  known  that  only  fresh  strawy 
manure,  which   contains   plenty  of  easily  fermentable 


XII.]      FERMENTATIONS  IN  THE  DUNG-HEAP        231 

urea  and  soluble  carbohydrates  in  the  straw,  will  get  up 
heat  quickly  and  serve  as  material  for  a  good  hotbed. 
A  gardener  is  also  accustomed  to  let  manure  in  this 
way  get  hot  and  turn  it  repeatedly  before  using  it  for 
such  purposes  as  the  growing  of  mushrooms.  Such  a 
process  the  gardener  calls  "taking  the  fire  out"  of  the 
manure,  whereby  he  means  that  he  has  got  rid  of,  in 
fact  burnt  up,  the  easily  fermentable  material,  and  at 
the  same  time  he  has  reduced  the  amount  of  ammonia, 
which  otherwise  might  easily  have  become  injurious  to 
the  roots  of  tender  plants.  The  fermentation  of  urea  is 
not,  however,  the  only  change  in  the  nitrogenous 
compounds  that  takes  place.  The  undigested  proteins 
in  the  faeces  and  similar  bodies  contained  in  the  litter 
are  attacked  by  the  putrefactive  bacteria ;  they  are 
resolved  into  simpler  substances,  and  may  eventually 
break  down  as  far  as  ammonia.  At  the  same  time, 
however,  certain  reverse  changes  take  place ;  the 
bacteria  which  develop  in  the  dung  are  so  enormous  in. 
number  that  they  take  for  themselves  an  appreciable 
percentage  of  the  soluble  compounds  of  nitrogen  there 
present  and  convert  them  into  insoluble  proteins 
forming  part  of  their  own  tissue.  The  various  changes, 
which  we  have  thus  indicated  as  taking  place  during 
the  making  of  farmyard  manure  while  it  is  still  under 
the  feet  of  the  animals,  are  also  continued  during  the 
storage  process.  After  the  manure  has  been  made  up 
into  a  mixen  or  dung-heap,  the  changes  become  much 
slower,  and  the  loss  of  nitrogen  is  comparatively  small  if 
the  heap  is  kept  moist  and  tightly  packed.  Losses  of 
nitrogen,  however,  are  constantly  occurring,  and  under 
ordinary  working  conditions  we  must  expect  that  only 
about  half  of  the  nitrogen  originally  contained  in  the 
food  finds  its  way  back  to  the  land  in  the  farmyard 
manure  ;  of  the  other  half  some  will  have  been  retained 


232  FARMYARD  MANURE  [chap. 

by  the  animal,  but  the  greater  part  will  have  been 
evaporated  as  ammonia  during  the  making  of  the 
manure  and  any  turning  it  may  have  received,  while 
some  will  have  been  set  free  as  nitrogen  gas.  At  the 
same  time  the  more  fermented  the  manure  becomes, 
and  the  shorter  and  more  rotten  its  condition,  the  less 
of  the  nitrogen  will  be  present  in  soluble  form,  and  the 
more  of  it  will  have  been  reconverted  into  substances 
akin  to  a  protein. 

In  making  farmyard  manure,  the  loss  of  valuable 
nitrogen  is  therefore  inevitable,  but  it  is  possible  to  keep 
the  loss  down  by  taking  suitable  precautions.  In  the 
first  place,  it  is  desirable  to  keep  the  manure  as  long  as 
possible  under  the  feet  of  the  animals.  The  least  loss 
occurs  when  the  manure  is  made  in  deep  boxes  in  which 
the  cattle  are  fed,  and  the  manure  is  not  removed  until 
it  is  ready  to  go  straight  out  on  the  land.  The  turning 
of  the  manure,  which  must  be  done  when  it  is  carted  out 
of  the  yard  in  order  to  form  the  mixen,  always  results  in 
loss.  Of  course,  another  source  of  loss  in  manure- 
making  arises  through  washing  or  leakage  of  the  liquid 
manure  away.  We  have  already  stated  that  the  most 
valuable  nitrogen  compounds  are  those  contained  in  the 
liquid  portions  of  the  manure,  which  is  also  rich  in 
potash,  so  that  if  this  material  is  allowed  to  drain  away, 
the  solids  that  are  left  behind  possess  but  little  value. 
The  dark  brown  liquid  which  we  often  see  oozing  from 
the  dung-heap  or  leaking  from  the  drains  of  a  badly 
kept  yard  constitutes  one  of  the  finest  of  fertilisers,  and 
in  the  management  of  a  yard  or  cattle  stalls  the  utmost 
care  should  be  taken  to  keep  this  material  soaked  up  in 
the  litter.  For  this  reason  a  partly  covered  yard  is 
desirable,  so  that  too  much  rain  is  not  allowed  to  wash 
through  the  manure.  On  the  other  hand,  the  wholly 
covered  yard  may  easily  let  the  manure  get  too  dry,  and 


XII.]      LOSS  OF  NITROGEN  IN  DUNG-MAKING        233 

so  result  in  a  large  evaporation  of  ammonia.  With  an 
entirely  covered  yard  it  is  often  necessary  to  pump  the 
liquid  over  the  litter  again  in  order  to  maintain  it  in 
a  proper  condition.  Various  materials  have  been  sug- 
gested to  reduce  the  loss  of  ammonia.  Gypsum,  super- 
phosphate, kainit,  and  other  substances  are  sometimes 
strewn  about  the  cattle  stalls  and  over  the  litter  with  the 
idea  of  absorbing  the  ammonia  whenever  it  is  set  free. 
All  these  substances,  however,  possess  but  little  practical 
value;  either  they  are  too  expensive,  or  they  set  up 
some  secondary  injurious  action  which  renders  them 
unsuitable.  The  only  practical  method  is  to  keep  the 
manure  tight  and  move  it  as  little  as  possible  during 
either  the  making  or  the  storage.  It  has  been  shown 
that  the  loss  on  storage  can  be  reduced  by  making  a 
foundation  to  the  new  dung-heap  of  a  few  inches  of  old 
and  well-rotted  manure. 

It  will  thus  be  seen  that  the  changes  during  the 
making  of  farmyard  manure  are  of  a  very  complex 
character.  In  the  first  place,  we  have  the  purely  carbon 
compounds  of  the  litter  turning  into  humus — this  change 
being  accompanied  by  a  loss  of  nearly  half  of  the  dry 
matter.  Secondly,  the  nitrogenous  compounds  are 
being  broken  down  in  one  or  several  stages  into  the 
form  of  ammonia,  some  of  which  escapes.  Lastly,  other 
bacterial  changes  under  certain  conditions  of  free 
aeration  causes  part  of  the  nitrogen  to  be  lost  by  con- 
version into  gas,  while  at  all  times  a  certain  amount  of 
reverse  change  from  the  soluble  to  the  insoluble  protein 
state  is  taking  place  through  the  multiplication  of  the 
bacteria  themselves.  No  preservatives  are  of  any  avail, 
but  the  loss  of  nitrogen  can  be  best  reduced  by  moving 
the  manure  as  little  as  possible  and  getting  it  on  to  the 
land  at  the  earliest  available  time.  With  these  general 
principles   in   mind   we  may  now  consider  a  little  the 


234 


FARMYARD  MANURE 


[chap. 


composition  of  farmyard  manure  as  met  with  in  practice. 
It  is  naturally  extremely  variable  according  to  the 
nature  of  the  animal,  and  of  the  food,  and  again 
according  to  the  method  of  making  adopted,  and  the 
age  of  the  product.  The  average  of  a  large  number  of 
analyses  at  Rothamsted  would  show  that  ordinary 
farmyard  manure  contains  about  three-quarters  of  its 
weight  of  water,  about  6  parts  per  thousand  of  nitrogen, 
2  to  3  of  phosphoric  acid,  and  3  to  4  of  potash,  or 
about  15  lb.  of  nitrogen,  5  lb.  of  phosphoric  acid,  and 
7  lb.  of  potash  per  ton.  Thus,  farmyard  manure  is  in 
the  main  a  nitrogenous  fertiliser,  and  as  an  all-round 
manure  it  is  somewhat  deficient  in  phosphoric  acid  for 
the  majority  of  crops.  The  effect  of  feeding  and 
management  is  well  seen  in  the  first  four  analyses  given 
in  Table  XXIV. 


Table  XXIV.— Composition  of  Farmyard  Manure. 


Nitrogen. 

Water. 

Phos- 
phoric 

Potash. 

In- 

Acid. 

Total. 

Soluble. 

soluble. 

I.   Made   from    Roots  and 

Hay  only    . 

747 

0-59 

0-l8 

0.41 

... 

... 

2.  Made   from    Roots   and 

Hay  and  Cake    . 

74-5 

0-82 

0-41 

0-41 

... 

3.  Made  from  Roots 'j       ( 

and  Hay      .         •  I  Ji  J 

78-0 

0-47    ;   o-o6 

0.41 

0-20 

0-37 

4.  Made  from  Roots  (2  | 

and  Hay  and  Cake  J  "^  I 

75-7 

0-69 

0-15 

0-54 

0.52 

0.48 

5.  Fresh  long  Straw  . 

66-2 

0-54 

0.32 

0-67 

6.             „            „         after 

rotting 

75-4 

o-6o 

... 

0-45 

0-49 

7.  Very  old        .        . 

53-1 

o-8o 

... 

... 

0.63 

0-67 

8.  London— Peat  Moss     . 

77-8 

0-88 

0-51 

0-37 

0-37 

1-02 

9.         „          Straw    . 

70-0 

0-62 

O-IO 

0.52 

0-48 

0-59 

No.  I  shows  that  fresh  manure  taken  from  under  the 
feet  of  the  animals  contained  about  0-59  per  cent,  of 


XII.]  RICH  AND  POOR  MANURE  235 

nitrogen,  of  which  70  per  cent,  was  in  an  insoluble 
condition,  when  the  animals  had  been  fed  upon  roots 
and  hay  only.  When,  however,  linseed  and  cotton  cake 
had  been  used  in  addition,  the  percentage  of  nitrogen 
had  risen  to  0-82  per  cent,  of  which  less  than  half  was  in 
the  insoluble  state.  These  two  analyses  refer  to 
manure  taken  straight  from  the  boxes ;  the  next  two 
refer  to  manure  made  in  exactly  the  same  way,  but 
thrown  out  into  the  mixen  and  stored  for  a  month  to  six 
weeks  before  being  analysed.  It  will  be  seen  that  where 
roots  and  hay  only  had  been  fed,  the  manure  then 
contained  much  less  total  nitrogen,  only  0-46  per  cent., 
though  the  amount  of  insoluble  nitrogen  is  about  the 
same  as  before,  thus  showing  that  the  losses  had  been 
falling  chiefly  upon  the  soluble  nitrogen.  Where  cake 
has  been  fed  the  total  loss  of  nitrogen  due  to  storage  had 
also  been  great,  and  had  again  fallen  upon  the  soluble 
portions  of  the  manure ;  for  whereas,  in  the  fresh 
manure  more  than  50  per  cent,  of  the  nitrogen  was  in 
the  soluble  state,  in  the  stored  manure  little  more  than 
20  per  cent,  existed  in  that  state.  Thus  the  difference 
between  manure  made  by  animals  receiving  concentrated 
foods  and  those  getting  only  roots  and  hay,  lies  chiefly 
in  the  amount  of  ammonia  and  other  soluble  nitro- 
genous substances  the  dung  contains.  This  difference 
is  well  seen  in  the  effect  of  the  manure  upon  the  crops 
grown  with  it.  For  example,  it  was  found  in  the  field 
experiments  made  with  the  manures  of  which  the 
analyses  have  just  been  quoted,  that  whereas  the  manure 
from  the  poorly  fed  animals  brought  about  an  increase 
of  crop  amounting  to  about  30  per  cent.,  the  manure 
from  the  richly  fed  animals  gave  an  increase  of  over  80 
per  cent  In  the  second  year  after  the  application, 
however,  both  lots  of  manure  gave  much  the  same 
return,  the   manure   made   from  the  animals  receiving 


236  FARMYARD  MANURE  [chap. 

cake  only  showing  a  superiority  of  7  per  cent,  over  the 
other.  In  slow- acting  nitrogenous  constituents  the 
two  lots  of  manure  were  alike  ;  the  difference  chiefly  lay 
in  the  ammonia,  which  had  its  effect  in  the  first  year, 
but  was  not  effective  afterwards. 

Analyses  Nos.  5,  6,  and  7  show  the  change  of  com- 
position that  dung  undergoes  on  storage.  There  is  a 
gradual  increase  in  the  percentage  of  nitrogen,  because 
the  original  material  has  lost  carbon  compounds  more 
rapidly  than  nitrogen,  so  that  the  percentage  of  nitrogen 
in  the  remaining  material  shows  an  increase.  At  the 
same  time,  from  other  analyses  we  learn  that  in  very  old 
short  dung  but  little  of  the  nitrogen  remains  in  a 
soluble  form  or  combined  as  ammonia.  Analyses  8  and 
9  show  the  composition  of  the  manure  that  is  obtainable 
from  London,  in  the  one  case  made  with  peat-moss 
litter  and  in  the  other  with  straw.  The  peat-moss 
manure  is  not  only  richer  in  nitrogen  because  of  the 
nitrogen  in  the  peat  moss  itself,  but  it  retains  a  much 
higher  proportion  of  the  nitrogen  in  an  ammoniacal  state 
because  of  the  absorptive  power  of  the  peat  for 
ammonia. 

From  a  consideration  of  the  origin  of  the  losses  of 
nitrogen  which  take  place  during  the  making  of  dung, 
and  of  the  above  analyses,  a  good  deal  of  guidance  can 
be  obtained  as  to  the  practical  management  of  farmyard 
manure,  which  remains  the  fundamental  fertiliser  in 
the  ordinary  course  of  farming  in  this  country.  In  the 
first  place,  since  it  is  clear  that  the  most  valuable  part 
of  the  manure  resides  in  the  liquid,  far  more  care  should 
be  taken  to  preserve  this  than  is  usually  the  case. 
Whether  the  dung  is  made  in  boxes  or  in  yards,  there 
should  be  sufficient  depth  to  allow  the  manure  to 
accumulate  under  the  animal  for  the  whole  winter  if 
need  be,  and  the  floors  should  be  rammed  with  clay  to 


XII.]  THE  MANAGEMENT  OF  MANURE  237 

render  them  watertight.  Yards,  in  particular,  should 
be  so  constructed  that  the  accumulated  manure  is  not 
above  the  general  ground  line  outside,  in  which  case 
there  will  always  be  a  gradual  soaking  away  of  the 
liquid.  On  the  other  hand,  yards  made  thus  below  the 
general  ground  level  are  apt  to  flood  in  heavy  rain,  so 
that  the  excess  of  liquid  containing  the  soluble  part  of 
the  manure  has  to  be  run  off  to  waste  by  means  of  a 
drain ;  this  can,  however,  be  avoided  by  cutting  drains 
outside  to  keep  land  water  from  running  into  the  yard, 
and  by  seeing  that  all  the  surrounding  sheds  are 
properly  provided  with  guttering.  For  real  economy  of 
litter,  part  at  least  of  the  yard  should  be  covered  ;  if  the 
whole  yard  is  covered,  a  certain  amount  of  care  is 
necessary  to  prevent  the  dung  from  getting  at  times  too 
dry.  Only  just  enough  litter  should  be  used  to  soak  up 
the  urine,  and  in  order  to  prevent  the  liquid  working  up 
to  the  surface  with  the  trampling,  the  floor  of  the  yard 
should  run  down  to  a  slight  hollow,  filled  at  first  with 
something  stiff  like  bean  haulm  or  coarse  peat  moss,  in 
which  the  excess  of  liquid  may  collect.  Above  all,  the 
manure  should  be  kept  tightly  trampled ;  the  greatest 
amount  of  loss  takes  place  when  the  urine  falls  on  a  thin 
layer  of  loose  strawy  litter.  The  yards  and  boxes  should 
be  deep  enough  to  carry  the  animals  through  the  whole 
winter,  so  that  they  need  not  be  cleaned  out  except 
when  dung  is  wanted  to  go  straight  on  the  land.  A  box, 
for  example,  8  feet  by  10  feet  in  area,  with  an  available 
depth  of  3  feet,  would  hold  about  9  cubic  yards,  or  8 
tons  of  dung  when  well  trodden  down.  This  would 
accommodate  two  beasts,  each  receiving  10  lb.  of  straw 
in  food  and  12  lb.  in  litter  per  diem,  for  four  months. 
As  far  as  possible,  manure  in  the  spring  should  be  left 
undisturbed  until  the  autumn,  it  may  then  be  carted  out 
on  to  the  stubbles  and  ploughed  in  where  potatoes  or 


238  FARMYARD  MANURE  [chap. 

roots  are  to  be  taken  in  the  following  spring.  Even 
on  the  lightest  soils  the  land  will  be  more  benefited 
thus,  than  if  the  manure  is  made  up  into  a  mixen  and 
only  put  on  immediately  before  the  roots  are  grown. 
Sometimes,  of  course,  a  potato  grower  must  have  a 
supply  of  well -rooted  manure  to  put  in  the  drills 
immediately  before  planting ;  this  can  often  be  got 
from  the  lower  layers  of  the  earliest  used  boxes  or 
yards,  since  a  mixen  should  be  avoided  as  much  as 
possible.  The  principle  to  keep  in  mind  is  that  every 
disturbance  of  farmyard  manure  results  in  loss,  and  that 
the  shorter  the  time  which  elapses  between  the  dropping 
of  the  dung  and  its  application  to  the  land,  the  less  this 
loss  of  fertilising  material  will  become. 

In  considering  the  value  of  farmyard  manure  as  a 
fertiliser,  one  has  to  keep  in  mind  that  it  is  an  essential 
product  of  the  farm,  and  that  it  must  constitute  the 
main  source  of  manure  for  the  land  under  the  conditions 
of  ordinary  mixed  farming,  where  artificial  manures  will 
only  be  used  as  supplements  and  not  as  rivals.  It  is 
only  in  certain  special  cases,  such  as  potato  or  hop 
growing,  where  the  ordinary  course  of  farming  does  not 
supply  as  much  farmyard  manure  as  is  wanted,  that  the 
question  has  to  be  decided  whether  artificial  manures  or 
dung  from  the  towns  shall  be  purchased,  or  again, 
whether  stock  shall  be  fattened  solely  with  the  view  of 
making  manure. 

As  a  fertiliser,  the  chief  value  of  farmyard  manure 
lies  in  the  fact  that  it  contains  all  the  elements  of  a 
plant's  nutrition — nitrogen,  phosphoric  acid,  and  potash 
— though  for  a  well-balanced  manure  the  phosphoric 
acid  is  comparatively  deficient.  Moreover,  the  nitrogen 
is  present  in  various  forms  of  combination,  varying  from 
the  rapidly  acting  ammonia  compounds,  down  to  some 
of  the  undigested  residues,  which  will  remain  for  a  very 


XII.]  USE  OF  FARMYARD  MANURE  239 

long  period  in  the  soil  before  becoming  available  for  the 
plant.  In  consequence,  dung  is  a  lasting  manure,  which 
accumulates  in  the  soil  to  build  up  what  a  farmer  calls 
"high  condition" — the  state  of  affairs  which  prevails 
when  the  reserves  of  manure  in  the  soil  are  steadily  and 
continuously  passing  into  the  available  condition  in 
sufficient  amount  for  the  needs  of  the  crop,  so  that 
there  is  no  necessity  for  freshly  applied  active  manure — 
a  mode  of  nutrition  which  results  in  healthy  growth  and 
good  quality.  But  however  marked  the  farmer's  prefer- 
ence is  for  such  lasting  manures,  the  delay  in  realising 
the  capital  they  represent  means  a  certain  amount  of 
loss ;  besides  which,  some  of  the  constituents  of  farm- 
yard manure  are  so  slowly  acting  as  to  be  hardly 
recoverable  during  the  lifetime  of  the  tenant. 

An  examination  of  the  records  at  Rothamsted  show^ 
that  when  farmyard  manure  was  put  on  at  the  rate  of 
14  tons  per  acre  every  year  for  the  wheat  crop,  at  the 
end  of  fifty  years  only  26  per  cent  of  the  nitrogen  applied 
had  been  recovered  in  the  crop,  and  less  than  20  per  cent, 
remained  stored  up  in  the  soil ;  more  than  half  had 
been  wasted  either  by  bacterial  processes  giving  rise  to 
nitrogen  gas  in  the  soil,  or  by  the  washing  out  of  nitrates 
into  the  drains.  This,  of  course,  is  a  very  extreme  case, 
both  because  wheat  is  a  plant  taking  a  comparatively 
small  amount  of  nitrogen  out  of  the  soil,  and  also 
because  the  land  had  become  so  laden  with  farmyard 
manure,  that  all  the  processes  of  bacterial  decay  and 
destruction  of  the  nitrogenous  compounds  had  been 
increased  far  beyond  the  normal.  On  the  mangold  plot, 
about  32  per  cent,  of  the  nitrogen  in  the  dung  has  been 
recovered  when  the  dung  had  been  put  on  year  by  year, 
but  when  the  manure  was  only  put  on  once  in  four  years, 
about  three-quarters  of  the  total  nitrogen  applied  was 
recovered  in  the  four  crops  grown  with  that  manure  and 


240  FARMYARD  MANURE  [chap. 

no  other  fertiliser.  Other  experiments  at  Rothamsted 
show  how  lasting  is  the  effect  of  farmyard  manure,  ie. 
how  very  slowly  some  of  the  nitrogenous  compounds 
get  oxidised  by  bacteria  and  converted  into  a  form 
available  for  the  plant.  In  two  cases,  on  the  grass  field 
and  on  the  barley  field,  farmyard  manure  at  the  rate  of 
14  tons  to  the  acre  was  applied  for  eight  years  and 
then  discontinued.  The  effect  of  that  application  can 
still  be  traced  more  than  forty  years  later,  in  the  higher 
crop  given  by  these  manured  plots  than  by  the  plots 
which  had  remained  unmanured  the  whole  time.  Of 
course,  these  long-continued  effects  of  farmyard  manure 
are  not  great  in  themselves,  and  will  only  be  perceptible 
when  the  land  has  been  reduced  to  the  lowest  ebb  of 
fertility  by  continual  cropping  without  manure.  Since 
the  greater  part  of  the  nitrogen  contained  in  farmyard 
manure  is  not  in  a  condition  to  be  utilised  by  the  plant 
until  it  has  first  been  attacked  by  bacteria  and  converted 
into  ammonia  or  nitrates,  it  may  happen  that  the  plant, 
though  it  has  been  well  supplied  with  farmyard  manure, 
cannot  obtain  the  nitrogen  therein  quickly  enough, 
when  the  season  is  otherwise  favourable  to  growth. 
For  example,  we  see  from  the  Rothamsted  experiment 
with  mangolds  that  in  years  of  large  crop  the  plant 
which  receives  some  active  source  of  nitrogen  like 
nitrate  of  soda  will  grow  heavier  crops  than  those  which 
receive  farmyard  manure  alone ;  although  the  amount 
of  nitrogen  in  the  manure  applied  to  the  latter,  in 
addition  to  that  which  is  already  stored  up  in  the  soil, 
may  be  far  in  excess  of  the  plant's  requirements.  From 
many  experiments  it  can  be  demonstrated  that  when 
the  grower  is  aiming  at  a  very  large  crop,  for  example 
of  potatoes  or  mangolds,  it  is  more  economical  to  obtain 
this  by  using  a  mixture  of  dung  and  active  fertilisers 
than  by  increasing   the  amount  of  dung  alone.     The 


XII.]  PHYSICAL  EFFECTS  OF  FARMYARD  MANURE  241 

slowness  with  which  dung  can  yield  up  its  contents  of 
nitrogen  is  particularly  noticeable  in  the  early  spring, 
when  the  land  is  cold  and  bacterial  actions  in  conse- 
quence go  on  very  slowly.  However  rich  the  soil  may 
be  in  farmyard  manure  and  its  residues,  the  gardener 
who  is  in  a  hurry  to  push  on  early  production,  finds  it 
very  necessary  to  use  an  immediately  active  nitrogenous 
manure  like  nitrate  of  soda. 

The  value  of  farmyard  manure  to  the  land  is  by  no 
means  confined  to  its  fertilising  action  ;  its  physical 
effects  upon  the  texture  and  water-holding  powers  of 
the  soil  are  equally  important ;  indeed,  for  some  crops, 
and  particularly  in  droughty  seasons,  these  factors  count 
for  more  than  fertilisers  towards  ensuring  a  good  yield. 
The  farmyard  manure,  as  it  rots  down  in  the  soil,  goes 
to  restore  the  stock  of  humus,  which  otherwise  is  always 
tending  to  oxidise  and  diminish,  and  the  humus, 
considered  merely  from  the  physical  side,  contributes 
largely  to  the  fertility  of  the  soil.  In  the  first  place,  it 
improves  the  texture  of  all  soils ;  to  sands  it  gives 
cohesion  and  water-retaining  power,  while  by  loosely 
binding  together  the  finest  particles  of  clay  soils  it 
renders  them  more  porous  and  friable.  When  a  piece 
of  old  grass  land,  even  on  the  stiffest  of  soils,  has  been 
ploughed  up,  it  is  easy  to  see  the  beneficial  effect  of  the 
humus  that  has  been  accumulated  ;  after  the  winter  the 
plough  slice  will  crumble  naturally  so  as  to  harrow  down 
at  once  to  a  mellow  seed  bed,  whereas  a  neighbouring 
piece  of  the  same  soil  that  has  long  been  under  arable 
cultivation  will  only  show  a  number  of  harsh,  intractable 
clods.  The  importance  of  a  good  seed  bed  to  the  future 
well-being  and  ultimate  yield  of  the  crop  can  hardly  be 
exaggerated ;  it  is  the  basis  of  all  good  farming,  so  that 
even  when  the  fertilising  properties  of  farmyard  manure 
have  been   replaced   by  artificial  manures,  some  other 

Q 


242  FARMYARD  MANURE  [chap. 

means,  such  as  the  ploughing-in  of  green  crops,  must  be 
employed  in  order  to  maintain  the  stock  of  humus. 
The  effect  of  dung  upon  the  soil  is  seen  in  two  ways. 
In  the  first  place,  it  will  cause  the  surface  soil  to  absorb 
more  of  the  rainfall  and  to  hold  it  up  near  the  surface, 
so  that  cases  may  be  found  in  which  the  subsoil  is 
actually  drier  where  farmyard  manure  has  been  used, 
because  the  rain  has  been  held  near  the  surface  and 
therefore  within  reach  of  evaporation.  But  the  chief 
value  of  the  humus  which  the  farmyard  manure  con- 
tributes to  the  soil  lies  in  the  better  texture  that  it 
induces.  It  is  particularly  noticeable  in  the  number  of 
plants  that  are  obtained  when  growing  turnips  and  other 
root  crops.  When  soils  have  long  been  farmed  without 
organic  manure,  however  rich  they  may  be  in  the  con- 
stituents of  plant  food,  there  will  be  great  difficulties  and 
even  total  failures  in  obtaining  a  proper  stand  should 
the  weather  conditions  have  been  unsuitable  soon  after 
sowing. 

In  ordinary  mixed  farming,  undoubtedly  the  best 
way  of  utilising  farmyard  manure  is  to  apply  it  to  the 
root  crops,  and  especially  to  mangolds  and  potatoes. 
Swedes  require  much  less  nitrogen  than  do  the  other 
root  crops.  They  also  require  a  firm  but  fine  tilth ;  in 
consequence,  not  more  than  lo  to  12  tons  of  dung  per 
acre  should  be  given  for  swedes,  and  it  should  be  applied 
in  the  autumn,  in  order  that  it  may  become  well  rotted 
down  before  the  spring  cultivation  begins.  But  up  to 
20  tons  of  dung  per  acre  can  be  profitably  employed  for 
mangolds  and  potatoes,  and  it  can,  if  necessary,  be 
applied  immediately  before  sowing.  In  America  where 
corn  (maize)  takes  the  place  of  root  crops,  the  farmyard 
manure  may  most  profitably  be  applied  to  that  crop.  Any 
surplus  dung,  after  the  requirements  of  the  root  crops 
have  been  satisfied,  is  probably  best  given  to  the  young 


XII.]  MANURING  BY  FEEDING  243 

seeds  in  the  early  winter,  to  act  both  as  a  fertiliser  and 
as  a  mulch.  The  seeds  benefit  greatly,  and  at  the  same 
time  much  of  the  added  fertility  is  retained  for  the  corn 
crop  that  follows  ;  manuring  the  young  seeds  is  certainly 
preferable  to  the  very  general  custom  of  manuring  the 
old  ley  before  it  is  ploughed  up  for  wheat  or  oats.  A 
certain  amount  of  the  farmyard  manure  made  on  the 
farm  should,  however,  always  be  reserved  for  the 
meadow  land,  especially  on  light  soils  and  on  land 
comparatively  newly  laid  down  to  grass.  Of  course, 
dung  would  be  wasted  on  rich  grazing  land ;  it  is  the 
thin  light  soils  that  are  cut  for  hay,  or  grass  land  that 
has  only  been  laid  down  for  a  few  years  and  has  had  no 
time  to  accumulate  a  stock  of  humus,  which  are  most 
benefited  by  an  occasional  dressing  of  farmyard  manure 
— once  in  every  four  or  five  years. 

In  Great  Britain  many  farmers  use  but  little  fertiliser 
for  their  land  beyond  the  farmyard  manure  that  they 
make,  but  this  farmyard  manure  contains  not  only  the 
fertilising  constituents  which  have  been  drawn  from 
the  soil  of  the  farm  and  are  contained  in  the  roots  and 
hay  fed  to  the  stock  and  the  straw  that  has  been 
trampled  down  as  litter,  but  they  also  enrich  the  dung 
by  the  consumption  of  imported  feeding  stuffs,  such  as 
oilcakes,  maize,  and  feeding  meals,  etc.  It  therefore 
becomes  a  question  of  some  importance  to  determine 
what  extra  value  is  imparted  to  the  manure  by  these 
purchased  feeding  stuffs.  Specially  is  this  important 
when  the  tenant  is  about  to  leave  a  farm,  when  the 
dung  that  has  been  made  during  the  last  year  of  his 
tenancy  still  remains  in  the  yard  and  has  not  produced 
a  crop.  If  purchased  linseed  cake,  cotton  cake,  maize, 
and  similar  foods  have  been  fed  to  the  stock  which 
made  the  manure,  there  will  be  left  on  the  farm  nitrogen 
and  other  valuable  constituents  which  are  just  as  much 


244  FARMYARD  MANURE  [chap. 

unused  fertilisers  as  if  they  were  contained  in  bags  still 
unopened  from  the  manufacturer.  We  have  seen  in  the 
analysis  that  the  manure  made  from  rich  feeding  stuffs 
is  very  superior  to  that  made  from  low-grade  materials 
like  roots  and  hay  alone.  From  the  records  of  the 
experiments  upon  fattening  animals,  we  can  obtain 
some  idea  of  how  much  nitrogen  contained  in  the  food 
eventually  reaches  the  soil.  We  have  seen,  for  example, 
that  the  animal  only  retains  perhaps  lo  per  cent,  of 
that  which  was  given  to  it  in  the  food,  but  that  in 
making  the  manure,  considerable  losses  set  in  even 
under  the  best  conditions — losses  due  to  the  volatilisation 
of  ammonia  and  to  the  setting  free  of  nitrogen  gas  by 
bacteria.  Though  these  losses  are  variable,  and  depend 
both  upon  the  nature  of  the  stock  and  the  care  taken  in 
managing  the  manure,  we  can  assume  as  a  working 
compromise  that  half  of  the  nitrogen  contained  in  the 
food  fed  to  the  different  classes  of  animals  upon  the 
farm  will  eventually  reach  the  land  again.  Of  the 
nitrogen  and  phosphoric  acid,  the  losses  are  confined 
to  the  proportion  retained  by  the  animal  and  whatever 
may  be  lost  by  drainage,  but  we  may  again  assume 
that  about  three-quarters  of  the  phosphoric  acid  and  all 
of  the  potash  contained  in  the  food  will  come  back  to 
the  land.  Acting  upon  these  assumptions,  we  may 
proceed  to  calculate  the  value  of  the  materials  which 
the  consumption  of  a  ton  of  any  given  food-stuff  will 
add  to  the  manure  made  during  its  consumption.  To 
take  a  concrete  example — the  decorticated  cotton-seed 
cake  contains  about  6-9  per  cent,  of  nitrogen,  and  we 
assume  that  half  of  this  (3-45  per  cent.)  finds  its  way 
into  the  manure.  If,  on  the  principle  explained  before, 
we  reckon  that  the  unit,  i.e.  i  per  cent,  of  nitrogen,  is 
worth  I2S.,  then  the  value  of  the  nitrogen  added  to  the 
dung  by  the  ton  of  cake  will  be  £2^  is.  6d.     The  cake 


XII.]  COMPENSATION  FOR  PURCHASED  STUFFS   245 

also  contains  3-1  per  cent,  of  phosphoric  acid,  three- 
quarters  of  this  at  3s.  a  unit  would  amount  to  7s. ;  lastly, 
there  is  2  per  cent,  of  potash,  which  at  4s.  a  unit  would 
add  8s.  to  the  value  of  the  manure.  The  total  value, 
then,  that  the  consumption  of  a  ton  of  decorticated 
cotton  cake  adds  to  the  manure  is  ;£"2,  is.  6d.  +  7s.  +  8s. 
=  £2^  1 6s.  6d.  Again,  in  the  case  of  maize  there  is  only 
1-7  per  cent,  of  nitrogen  in  the  food,  the  value  of  half  of 
which  at  12s.  a  unit  would  be  los. ;  there  is  0-6  of 
phosphoric  acid,  three-quarters  of  which  would  add 
IS.  6d.,  and  there  is  037  per  cent,  of  potash,  which  would 
add  a  further  is.  6d.,  making  13s.  in  all  as  the  value 
added  to  the  manure  by  the  consumption  of  a  ton  of 
maize.  It  will  be  seen  that  these  two  foods  differ  very 
greatly  in  the  value  of  the  manurial  residues  they  leave 
behind  when  they  have  been  consumed  by  stock, 
the  maize  being  a  food  rich  chiefly  in  oil  and  carbo- 
hydrates, which  possess  no  fertilising  value.  The  value 
of  these  foods  for  manure-making  purposes  again  bears 
no  relation  to  their  cost,  which  is  determined  by  the  oil 
and  carbohydrates  present  as  much  as  by  the  proteins. 
The  figures  that  we  have  thus  deduced  for  the  addition  of 
fertilising  material  to  the  dung  due  to  the  consumption 
of  purchased  foods,  represent  what  is  sometimes  called 
the  compensation  values  to  be  attached  to  these  foods, 
because  they  represent  the  price  which  should  be  paid 
to  a  tenant  leaving  the  farm  for  the  purchased  food- 
stuffs which  had  been  consumed  during  the  last  year 
of  his  tenancy,  from  the  manure  made  by  which,  of 
course,  he  has  as  yet  reaped  no  benefit.  They  are, 
however,  we  have  said  before,  only  rough  approxima- 
tions to  the  real  truth.  On  many  farms  the  farmyard 
manure  is  so  neglected  that  the  loss  of  nitrogen  becomes 
much  more  than  the  50  per  cent,  that  we  have  assumed, 
and  the  longer  the  manure  remains  out  of  the  land  and 


246  FARMYARD  MANURE  [chap. 

lies  about  in  the  yard,  the  greater  will  this  loss  become. 
On  the  other  hand,  when  food  is  fed  upon  the  land  and 
is  not  used  for  making  manure  in  yards  or  stalls,  as, 
for  example,  when  the  sheep  are  folded  on  the  land  or 
cattle  are  given  cotton  cake  while  they  are  grazing,  the 
losses  of  nitrogen  would  be  much  reduced,  because 
the  urine  which  contains  the  valuable  part  of  the 
nitrogen  is  at  once  absorbed  by  the  soil,  and  none  of 
the  usual  wasteful  actions  are  set  up.  On  the  other 
hand,  a  farm  producing  much  milk  takes  rather  more, 
both  of  phosphoric  acid  and  nitrogen,  out  of  the  food  than 
the  proportions  we  have  been  assuming,  so  that  dung 
made  by  milch  cows  is  never  very  rich.  But  taking 
all  these  things  into  account,  we  shall  not  be  far 
wrong  in  assuming  a  loss  of  half  of  the  nitrogen  under 
the  ordinary  conditions  of  mixed  farming,  and  in 
thereby  deducing  compensation  values  in  the  manner 
we  have  set  up  above. 

We  can  apply  the  same  principles  to  obtain  the 
value  of  a  ton  of  farmyard  manure.  Of  course,  as  a 
rule,  the  farmyard  manure  is  a  normal  product  of  the 
farm,  and  the  only  problem  is  to  make  it  as  carefully  as 
possible,  and  apply  it  to  the  best  purpose  afterwards. 
But  there  are  occasions,  especially  in  growing  of  crops 
like  market  garden  produce,  potatoes,  hops,  when  it 
becomes  a  question  of  whether  it  is  more  profitable  to 
buy  farmyard  manure  or  artificial  fertilisers,  or  to  keep 
stock  in  order  to  make  the  farmyard  manure  that  is 
required.  It  will  be  found  that  if  the  farmyard  manure 
account  is  charged  with  the  litter,  and  the  compensation 
values  of  the  foods  calculated  in  the  fashion  described 
above,  on  the  assumption  that  half  of  the  nitrogen, 
three-quarters  of  the  phosphoric  acid,  and  all  the  potash 
contained  in  the  food  found  its  way  into  the  manure, 
then  the  farmyard  manure  will  cost  from  8s.  6d.  to  12s. 


XII.]  COST  OF  FARMYARD  MANURE  247 

the  ton  by  the  time  it  is  ready  to  go  on  the  land ;  the 
difference  in  price  representing  a  real  difference  in  value, 
according  to  whether  much  concentrated  food  has  been 
consumed  during  its  manufacture  or  not.  The  farmer, 
then,  who  is  faced  with  the  problem  of  specially  making 
farmyard  manure,  or  on  the  other  hand,  buying  artificial 
fertilisers  or  town  dung,  ought  to  reckon  that  such 
manure  of  fair  quality  will  cost  about  los.  a  ton  to 
make.  In  experiments  dealing  with  fertilisers,  this 
figure  should  be  taken  as  an  average  valuation  of  well- 
made  farmyard  manure.  It  is  noticeable  also  that  if 
we  proceed  to  a  valuation  of  farmyard  manure  on  the 
basis  of  the  fertilising  constituents  it  contains  at  the 
usual  unit  rates,  its  price  would  work  out  to  about 
the  same  figure  of  los.  a  ton. 


CHAPTER  XIII 

ARTIFICIAL  MANURES  AND   FERTILISERS 

Nature  of  a  Fertiliser.  Fertilisers  containing  Nitrogen — Nitrate 
of  Soda,  Sulphate  of  Ammonia,  Soot — their  Use  and  Value. 
Fertilisers  containing  Phosphoric  Acid.  Bones,  Superphos- 
phate or  Acid  Phosphate,  Basic  Slag  or  Phosphate  Powder, 
Ground  Rock  Phosphate.  Potash  Fertilisers.  Guanos. 
Industrial  Residues.  Tankage.  Action  of  Fertilising 
Ingredients  upon  Crops.  Expenditure  on  Fertilisers. 
Character  of  Fertiliser  required  for  particular  Crops. 
Valuation  of  Fertilisers. 

It  has  already  been  explained  that  of  the  elements 
found  in  a  plant,  a  certain  number  are  absolutely 
essential  to  its  development,  but  that  most  of  these  are 
ordinarily  to  be  found  in  the  soil  in  sufficient  quantities 
for  the  plant's  requirements.  There  are,  in  fact,  only 
three  of  these  substances  in  which  the  soil  shows  any 
deficiency,  and  the  bodies  which  we  call  manures  or 
fertilisers  are  substances  containing  one  or  more  of 
these  three  elements — nitrogen,  phosphorus,  or  potash. 
At  the  present  time  the  terms  artificial  manure  and 
fertiliser  are  used  indifferently  to  indicate  such  com- 
mercial materials  as  are  of  value  to  the  plant,  whatever 
their  origin  ;  from  the  farmer's  point  of  view  an  artificial 
manure  is  any  concentrated  plant  food  which  he 
purchases  and  receives  in  bags.  In  reviewing  these 
substances  we  must  begin  by  drawing  a  distinction 
between  the  fertilisers  which  contain  only  one  ingredient 

248 


CHAP.  XIII.]       ESSENTIAL  CONSTITUENTS  249 

of  value  to  the  plant — the  single  manures,  as  we  may 
call  them — and  the  compound  manures  which  contain 
two  or  more  fertilising  ingredients.  We  may  again 
divide  these  materials  into  nitrogenous,  phosphoric,  and 
potassic  manures,  according  to  the  elements  which 
predominate  in  them.  The  next  most  important  factor 
to  be  taken  into  account  is  the  actual  percentage  of 
fertilising  material  which  the  manure  contains.  With 
certain  exceptions  to  be  dealt  with  later,  i  lb.  of 
nitrogen  possesses  the  same  value  in  whatever  fertiliser 
it  may  be  contained,  so  that  we  must  compare  all 
nitrogenous  manures  on  the  basis  of  the  amount  only 
of  nitrogen  they  contain.  Thus,  if  sulphate  of  ammonia 
contains  20  per  cent,  of  nitrogen,  and  soot  only  4  per 
cent.,  then  sulphate  of  ammonia  is  five  times  as  valuable 
as  the  soot,  if  we  leave  out  of  account  certain  other 
considerations  which  may  give  an  extra  value  to  one 
or  other  of  these  substances.  In  the  case  of  sulphate  of 
ammonia,  only  the  20  per  cent,  of  nitrogen  possesses  any 
value  to  the  soil  or  plant,  the  remaining  four-fifths, 
however  indispensable  to  the  constitution  of  the  manure, 
must  be  regarded  as  surplusage.  It  should  not,  however, 
be  considered  that  there  is  any  necessary  waste,  though 
the  manure  does  only  contain  a  proportion  of  the 
pure  fertilising  constituent,  for  these  bodies  must  be 
combined  to  be  of  any  use.  Pure  nitrogen  gas,  for 
example,  as  we  have  already  stated,  exists  in  air  in 
enormous  quantities,  but  is  of  no  value  to  the  plant 
until  it  has  been  brought  into  combination.  Pure 
phosphoric  acid,  again,  though  it  does  exist  and  would 
act  as  a  fertiliser,  is  a  very  scarce  material,  of  interest 
only  in  the  laboratory.  Thus  the  surplusage  which 
may  be  present  in  a  manure,  in  addition  to  its  percentage 
of  nitrogen  or  phosphoric  acid,  must  be  regarded  as  the 
vehicle  necessary  to  carry  these  valuable  constituents. 


250  ARTIFICIAL  MANURES  [chap. 

Among  the  fertilising  constituents  nitrogen  must  be 
given  the  first  place.  Not  only  is  combined  nitrogen 
much  more  expensive  than  either  potash  or  phosphoric 
acid,  I  lb.  costing  about  6d.  (12  cents),  but  as  a 
fertiliser  it  seems  to  have  a  much  more  direct  and 
immediate  action  upon  a  plant  than  the  other  two 
substances,  which  are  at  bottom  equally  indispensable. 
We  find  that  nitrogen  pushes  on  the  first  vegetative 
development  of  the  plant,  promotes  its  growth  in  fact, 
thus  enabling  it  to  search  the  ground  more  thoroughly 
for  phosphoric  acid  and  potash  which  may  there  be 
present.  For  some  little  time,  indeed,  crops  may  be 
grown  by  the  aid  of  nitrogenous  fertilisers  alone,  and 
there  is  always  some  tendency  to  use  an  excess  of  these 
substances  in  comparison  with  manures  containing 
phosphoric  acid  and  potash,  which  make  less  immediate 
show.  Of  the  purely  nitrogenous  fertilisers  there  are 
five  which  are  commonly  employed ;  sulphate  of 
ammonia,  the  most  concentrated,  contains  about  20  per 
cent,  of  nitrogen,  and  nitrate  of  soda  contains  about  15^ 
per  cent.  To  these  concentrated  fertilisers  two  new 
rivals  have  recently  arisen  in  the  shape  of  calcium 
cyanamide  or  nitrolim,  containing  about  18  per  cent,  of 
nitrogen,  and  nitrate  of  lime,  containing  about  1 3  per 
cent.  These  latter  materials  are  manufactured  artificially, 
the  nitrogen  they  contain  being  derived  from  the 
atmosphere.  Lastly  comes  soot,  a  waste  material  which 
contains  a  small  quantity  of  ammonia,  showing  from  i 
to  5  per  cent,  of  nitrogen  according  to  its  purity  and  the 
nature  of  the  material  from  which  it  has  been  derived. 

Sulphate  of  ammonia  is  a  product  recovered  in 
the  manufacture  of  coal-gas,  coke,  and  other  industries 
involving  the  destruction  of  coal.  It  is  a  pale  grey, 
crystalline  substance  that  is  freely  soluble  in  water, 
though  it  remains  dry,  and  does  not  naturally  run  down 


XIII.]  SULPHATE  OF  AMMONIA  251 

into  a  liquid  by  absorption  of  water  from  the  air. 
Although  it  is  so  soluble  in  water,  it  can  be  applied  to 
the  soil  without  any  danger  of  washing  out,  because  it 
there  interacts  with  the  humus  and  clay,  and  is  converted 
into  substances  which  are  for  the  time  insoluble.  It 
should  not,  however,  be  used  as  a  manure  long  before 
the  plant  is  ready  to  take  it  up,  because  it  is  readily 
converted  by  the  bacteria  we  have  spoken  of  before  into 
nitrates  which  do  wash  out  of  the  soil.  Sulphate  of 
ammonia  is  therefore  best  employed  as  a  top-dressing 
or  for  root  crops,  in  which  case  it  is  put  in  the  soil  at  the 
time  of  year  when  there  is  very  little  danger  of  any 
washing  out.  It  is  found  by  experience  that  when  a 
concentrated  nitrogenous  fertiliser  is  needed,  sulphate  of 
ammonia  is  most  suitable  for  shallow-rooted  crops,  like 
barley,  swedes,  turnips,  potatoes.  On  chalky  soils  and 
on  very  heavy  clays  it  is  the  most  useful  nitrogenous 
fertiliser,  but  it  should  not  be  used  on  light  sands  nor  on 
peaty  and  other  soils  which  have  any  tendency  to  get 
sour.  Being  such  a  concentrated  fertiliser,  only  small 
quantities  are,  as  a  rule,  required.  It  should  be 
remembered  also  that  when  used  as  a  top-dressing  it 
will  scorch  and  kill  the  foliage  of  any  green  plant  on 
which  it  happens  to  rest.  This  is  not  because  it  is 
poisonous,  but  merely  because  any  soluble  salt  in  contact 
with  the  green  leaf  of  a  plant  will  draw  water  from  the 
tissues,  and  eventually  kill  the  leaf  by  so  doing.  For 
this  reason  sulphate  of  ammonia  mixed  with  sand  is 
often  used  to  kill  out  weeds  on  lawns ;  when  sprinkled 
over  the  lawn  the  soluble  material  lodges  in  the  crowns 
and  rests  on  the  broad  leaves  of  weeds  like  plantains, 
buttercups,  and  daisies,  eventually  killing  them,  whereas 
it  does  not  touch  the  upright  leaves  of  grasses  but  slips 
down  to  their  roots  and  acts  as  a  fertiliser.  If  a  satis- 
factory result  is  to  be  obtained,  however,  the  weather 


252  ARTIFICIAL  MANURES  [chap. 

must  remain  dry ;  should  rain  come  the  sulphate  of 
ammonia  dissolves,  washes  down  into  the  lawn,  and 
fertilises  weeds  and  grass  alike.  A  more  effective  plan 
is  to  choose  a  fine  morning  and  put  a  pinch  of  sulphate 
of  ammonia  into  the  crown  of  the  plantains  and  other 
flat-leaved  weeds. 

Nitrate  of  soda  is  a  substance  obtained  from  certain 
extensive  natural  deposits  in  Chili,  and  has  been 
brought  to  this  country  since  about  1835.  It  forms  a 
grey  or  pinkish  soluble  salt  which  easily  picks  up  water 
from  damp  air,  and  will  even  pass  into  a  liquid  state 
when  left  to  itself.  It  is  not  retained  in  any  way  by  the 
soil,  and  is  most  commonly  employed  as  a  top-dressing. 
Since  it  has  to  undergo  no  change  but  can  feed  the 
plant  directly,  it  is  the  most  active  of  all  fertilisers,  and 
is  particularly  effective  in  forcing  on  a  plant  into  very 
rapid  growth.  Because  of  its  immediate  availability,  it 
is  also  a  specially  valuable  manure  in  early  spring,  when 
the  natural  processes  producing  ammonia  and  nitrates 
are  so  slowed  down  by  the  cold,  that  even  in  rich  soils 
the  plant  is  not  obtaining  sufficient  nitrogenous  food. 
Thus,  nitrate  of  soda  is  particularly  useful  to  the 
market  gardener  who  needs  to  force  on  his  crops  rapidly 
or  to  get  them  growing  specially  early,  and  very  large 
quantities  are  thus  employed  with  profit.  Nitrate  of 
soda  is  particularly  valuable  as  a  top-dressing  for  wheat 
and  maize,  for  grass  land  that  is  being  laid  up  for  hay, 
for  mangolds,  and  for  cabbages.  On  heavy  soils  it  often 
forms  an  unsatisfactory  manure,  because  it  leaves  the 
land  in  a  state  of  bad  tilth,  very  wet  and  sticky  after 
rain,  and  then  drying  into  hard  clods.  In  such  cases  a 
mixture  of  equal  parts  of  sulphate  of  ammonia  and 
nitrate  of  soda  is  even  more  effective  than  either  separ- 
ately, and  does  not  interfere  with  the  texture  of  the  soil. 
It   is   because  of  this   bad   effect   upon  the   tilth   that 


XIII.]  NITRATE  OF  SODA  253 

nitrate  of  soda  gets  called  a  stimulant  or  a  scourge,  and 
is  considered  to  rob  the  land.  This,  however,  is  no 
more  the  case  with  nitrate  of  soda  than  with  any  other 
single  manure.  Whenever  crops  are  grown  with  nitrate 
of  soda  alone,  we  are  removing  in  the  crop  nitrogen, 
phosphoric  acid,  and  potash,  only  one  of  which  is  being 
replaced,  so  that  the  land  must  inevitably  become 
poorer  in  the  other  two  constituents.  But  when  nitrate 
of  soda  is  employed  with  the  potash  and  phosphoric 
acid  that  are  equally  required  by  the  plant,  the  fertility 
of  the  land  is  maintained  unimpaired,  as  may  be  seen 
from  the  Rothamsted  experiments,  where  crops  have 
been  grown  with  such  a  mixture  on  the  same  land  for 
nearly  seventy  years  without  showing  any  decline  in  the 
average  yield.  The  other  two  fertilisers,  nitrolim  and 
nitrate  of  lime,  are  not  so  widely  known.  Nitrolim 
behaves  in  much  the  same  way  as  sulphate  of  ammonia, 
but  is  slower  in  its  action,  and  ought  to  be  put  on  the 
land  before  a  crop  is  sown.  Nitrate  of  lime  is  very 
similar  to  nitrate  of  soda,  though  it  has  not  the  same 
injurious  effect  upon  the  texture  of  the  soil.  It  should 
be  mentioned  that  nitrate  of  soda  is  poisonous  to  stock, 
and  deaths  have  been  reported  through  animals  licking 
the  bags,  or  drinking  water  in  which  nitrate  of  soda  had 
been  dissolved. 

Soot,  the  other  manure  which  has  been  men- 
tioned as  containing  nitrogen  and  no  other  fer- 
tilising ingredient,  is  chiefly  used  as  a  top-dressing 
for  wheat  in  the  spring.  For  this  purpose  it  is  very 
valuable,  not  only  because  of  the  fertilising  effect 
of  the  small  amount  of  nitrogen  it  contains,  but  also 
because  it  helps  to  protect  the  wheat  from  the  attack 
of  slugs  and  snails,  which  are  very  active  at  that  time  of 
the  year.  Moreover,  the  dark  colour  the  soot  imparts 
to  the  soil  is  of  value,  because  it  causes  an  increased 


254  ARTIFICIAL  MANURES  [chap. 

absorption  of  the  sun's  rays,  and  on  sunny  days  will  raise 
the  temperature  of  the  soil  by  one  or  two  degrees. 
Soot  is  very  variable  in  composition,  and  cannot,  as  a 
rule,  be  purchased  with  any  guarantee  as  to  the  amount 
of  nitrogen  it  contains.  The  best  guide  is  its  lightness. 
A  good  sample  should  be  free  from  all  admixture  of 
cinders  and  similar  refuse,  and  should  not  weigh  more 
than  28  lb.  per  bushel. 

The  second  of  the  great  groups  of  fertilisers  is  made 
up  of  those  which  contain  phosphoric  acid  as  their 
valuable  constituent.  Of  these,  bones  in  various  forms 
constitute  the  oldest  and  still  among  the  most  widely 
used  of  fertilisers.  A  bone  consists  of  a  mineral  frame- 
work containing  phosphate  of  lime  mixed  with  a  little 
carbonate.  This  mineral  framework  we  can  see  left 
behind  if  we  put  an  ordinary  bone  into  the  fire  or  in  a 
muffle  furnace.  On  the  other  hand,  if  we  immerse  a 
similar  bone  in  dilute  hydrochloric  acid,  after  a  day  or 
two  all  the  mineral  matter  will  become  dissolved  in  the 
acid,  and  there  will  be  left  behind  the  cartilaginous 
framework  of  the  bone,  consisting  of  material  contain- 
ing nitrogen,  which  will  be  converted  into  gelatine  when 
heated  up  with  water  at  high  temperatures.  At  one 
time  the  bones  were  only  roughly  broken  up,  and  then 
proved  to  be  only  a  slow-acting,  if  effective,  fertiliser. 
Nowadays  it  is  customary  to  remove  as  much  of  the  fat 
as  possible,  and  then  grind  the  bones  to  a  fine  powder, 
which  is  sold  as  "  bone  meal."  Owing  to  the  toughness 
of  the  organic  matter  contained  in  the  bones,  the  bone 
meal  is  never  really  fine,  and  though  it  is  highly  valued 
on  pastures  and  on  some  of  the  lighter  arable  soils,  it  is 
comparatively  slow  in  its  action  and  unremunerative  in 
its  results. 

In  addition  to  bone  meal,  various  other  manures  are 
prepared  from  bones ;  sometimes  they  are  treated  with 


XIII.]  SUPERPHOSPHATE  255 

oil  of  vitriol,  and  yield  a  soluble  acid  phosphate,  similar 
in  its  action  to  the  superphosphate  to  be  dealt  with 
later.  These  dissolved  bones  or  vitriolised  bones,  as 
they  are  called,  form  rather  a  damp  mass,  which  is  not 
very  easily  sown  from  a  machine  ;  they  offer  no  special 
advantage  over  a  mineral  superphosphate  which  is 
cheaper,  even  after  making  allowance  for  the  nitrogen 
the  bone  manure  also  contains.  Again,  the  bones  are 
sometimes  submitted  to  the  action  of  superheated 
steam  before  grinding,  thus  taking  out  of  them  most  of 
the  material  containing  nitrogen  ;  the  resulting  steamed 
bone  flour  is  a  friable  powder  rich  in  phosphates  but 
containing  only  about  i  per  cent,  of  nitrogen,  and 
forming  a  valuable  manure  on  light  soils.  Bone 
manures,  however,  no  longer  possess  their  former 
importance  when  they  were  almost  the  only  source  of 
phosphates.  For  many  years  deposits  of  mineral 
phosphates  of  lime  have  been  worked  for  manurial 
purposes.  Sometimes  these  rock  phosphates  are 
ground  to  a  very  fine  powder,  which  is  practically 
insoluble  in  water  but  which  does  slowly  become 
available  to  the  plant  in  soils  rich  in  organic  matter  and 
well  provided  with  moisture.  In  the  United  Kingdom, 
however,  such  ground  rock  phosphates  are  rarely 
employed ;  as  a  rule,  the  mineral  is  treated  with  oil  of 
vitriol  and  converted  into  the  soluble  phosphate  known 
as  superphosphate  or  acid  phosphate.  Superphosphate 
of  lime,  which,  for  manurial  purposes,  was  invented  by 
the  late  Sir  J.  B.  Lawes,  is  the  most  widely  employed  of  all 
the  phosphatic  manures.  Being  soluble  in  water,  it 
becomes  disseminated  throughout  the  soil,  and  is  there 
reprecipitated  wherever  it  comes  in  contact  with 
particles  of  carbonate  of  lime  or  humus.  Thus  the 
surface  soil  gets  mixed  with  precipitated  phosphate  of 
lime  in  a  very  fine  state  of  division,  and  it  is  to  the  fact 


256  ARTIFICIAL  MANURES  [chap. 

that  this  precipitated  material  is  so  much  finer  and 
more  thoroughly  mixed  with  the  soil  than  any  ground 
insoluble  material  can  be,  that  the  activity  of  the  super- 
phosphate is  due.  Superphosphate  is  manufactured  in 
several  grades  of  concentration  ;  it  is  adapted  to  all  crops 
for  which  phosphatic  manure  is  wanted,  except  for 
turnips  on  an  acid  soil  where  "  finger-and-toe "  occurs. 
Nowadays  the  great  rival  to  superphosphate  is  a 
fertiliser  called  basic  slag,  which  is  obtained  as  a  by- 
product in  the  manufacture  of  steel  from  pig-iron, 
contaminated  with  phosphorus.  As  it  occurs  in 
commerce,  basic  slag  is  a  very  fine  dark  powder, 
insoluble  in  water,  containing  about  40  per  cent,  of 
phosphate  of  lime  and  also  a  certain  amount  of  free 
lime.  This  free  lime,  together  with  an  excess  of  loosely 
combined  lime,  gives  the  manure  its  basic  character ;  it 
is  alkaline  in  contradistinction  to  the  acid  nature  of 
superphosphate,  and  it  adds  lime  to  the  soil,  whereas 
superphosphatic  takes  it  away.  The  phosphates  of 
basic  slag  are  insoluble  in  water,  but  they  are  easily 
attacked  by  the  carbon  dioxide  contained  in  the  soil 
water,  and  it  is  found  in  practice  that  they  are  readily 
available  to  plants,  especially  upon  soils  that  are  damp 
and  sour.  Basic  slag  has  proved  of  particular  value  as  a 
fertiliser  for  all  pastures  on  clay  land,  in  which  case  the 
free  lime  that  is  also  present  adds  greatly  to  the  value 
of  the  manure.  Its  value  as  a  fertiliser  on  arable  land 
of  a  light  and  dry  character  has  hardly  been  enough 
recognised  in  the  United  Kingdom.  Basic  slag  is  also 
sometimes  known  as  Thomas's  phosphate  powder  or 
basic  cinder.  It  should  be  carefully  distinguished  from 
the  ordinary  slag  of  ironworks,  which  contains  no 
phosphoric  acid  to  make  it  valuable.  Many  of  the 
compound  fertilisers  to  be  dealt  with  later  also  contain  a 
considerable  quantity  of  phosphoric  acid  ;  indeed  some 


XIII.]  POTASH  MANURES  257 

of  the  guanos  contain  so  little  else  that  they  may  be 
practically  regarded  as  phosphatic  manures,  whose 
action  is  very  similar  to  that  of  the  steamed  bone  flour 
and  bone  meal  mentioned  above. 

The  third  group  of  fertilisers  are  those  which  supply 
the  element  potash,  but  as  this  substance  is  naturally 
abundant  in  many  soils,  especially  those  containing  much 
clay,  the  need  for  potassic  fertilisers  is  not  so  much  recog- 
nised, and  they  are  less  generally  employed  by  farmers 
than  either  nitrogenous  manures  or  phosphates.  For  a 
long  time  the  only  source  of  potash  was  the  ashes  of 
wood  and  other  vegetable  matter  like  kelp,  but  these 
sources  have  been  entirely  superseded  by  the  opening 
up  during  the  last  half  century  of  enormous  mines  of 
potash  salts  near  Stassfurt,  in  Germany,  whence  the 
world's  supply  of  potash  is  now  almost  wholly  derived. 
Various  grades  of  manures  are  put  upon  the  market, 
the  most  common  being  an  impure  material  containing 
about  12  per  cent,  of  potash,  which  is  called  kainit, 
though  nowadays  it  has  little  right  to  the  title,  since  it 
consists  almost  entirely  of  chlorides  of  potash,  soda,  and 
magnesia.  As  a  rule,  kainit  is  just  as  valuable,  potash 
for  potash,  as  the  more  concentrated  fertilisers,  because 
the  salt  and  other  impurities  wash  out  of  the  soil 
without  doing  any  harm.  In  some  cases,  however,  it  is 
desirable  to  use  pure  materials,  and  sulphate  of  potash 
of  two  grades  of  purity,  and  muriate  of  potash,  the  most 
concentrated  of  all,  can  then  be  obtained.  There  is  a 
general  idea  that  with  crops  like  potatoes,  for  which 
potash  salts  are  largely  employed,  the  sulphates  give  rise 
to  better  quality  of  the  product  than  do  the  chlorides. 

Of  the  compound  fertilisers  the  oldest  and 
still  the  most  widely  employed  are  the  guanos,  of 
which  the  Peruvian  guanos,  obtained  from  some  of  the 
rainless  islands  off  the  west  coast  of  South  America,  are 

R 


258  ARTIFICIAL  MANURES  [chap. 

by  far  the  most  important.  These  guanos  consist  of 
the  excrements  of  the  sea-birds  which  frequent  these 
islands  in  enormous  numbers  during  the  breeding 
season.  Owing  to  the  absence  of  rain,  the  material 
accumulates  from  year  to  year  with  very  little  change,  and 
can  be  excavated  and  readily  reduced  to  a  fine  powder. 
It,  however,  undergoes  some  slow  process  of  decay  and 
washing,  so  that  while  the  recent  deposits  contain  as 
much  as  1 5  per  cent,  of  nitrogen  and  only  about  20  per 
cent,  of  phosphates,  in  the  oldest  deposits  the  nitrogen 
is  reduced  to  less  than  3  per  cent.,  while  the  phosphates 
have  risen  to  nearly  60  per  cent.  It  is  thus  very 
important  to  purchase  a  guano  according  to  its  analysis, 
and  not  by  its  name  alone.  In  any  case  these  fertilisers 
will  be  found  to  be  dearer  than  the  same  amount  of 
nitrogen  and  phosphoric  acid  in  other  forms,  the  extra 
price  representing  the  farmer's  long  experience  of  their 
kindly  action  upon  his  crops.  The  richer  guanos,  con- 
taining 6  per  cent,  of  nitrogen  and  upwards,  are 
extremely  active  fertilisers,  the  nitrogen  being  in  forms 
which  readily  get  converted  into  ammonia,  and  the 
phosphates  being  comparatively  soluble  in  water.  They 
are  also  very  safe,  well-balanced  manures,  containing 
nitrogen  and  phosphoric  acid  in  much  the  same  pro- 
portions as  are  required  by  plants,  and  also  a  little 
potash.  Moreover,  many  different  compounds  of 
nitrogen  are  present  which  differ  in  the  rate  at  which 
they  will  decay,  so  that  the  plant  is  fed  continuously,  and 
suffers  from  no  excess  of  available  nitrogen  in  the  soil  at 
any  time.  Such  a  fertiliser  leads  to  a  steady,  equable 
growth,  which  is  generally  attended  by  superior  quality 
in  the  product  Thus  the  Peruvian  guanos  become 
excellent  fertilisers  for  crops  where  the  quality  is  of 
importance,  and  where  it  is  not  necessary  to  cut  down 
the    expenditure    on    fertilisers   to    the    lowest    limit. 


XIII.]  MEAT  AND  FISH  MANURES  259 

Somewhat  similar  in  their  composition  and  in  their 
action  to  the  Peruvian  guanos  are  a  number  of  manures 
which  are  manufactured  out  of  residues  of  meat  and 
fish  that  accumulate  in  various  processes  of  preserving, 
canning,  and  packing  these  articles  for  food.  It  is 
customary  to  extract  as  much  as  possible  of  the  fat  of 
these  materials,  and  then  reduce  them  to  a  very  fine 
powder,  which  will  contain  nitrogen  varying  from  4  to 
10  per  cent,  and  phosphates  which  lie  between  10  and  50 
per  cent.  These  fertilisers  decay  in  the  soil,  and  yield 
ammonia  in  the  same  steady,  continuous  fashion  as  the 
nitrogen  compounds  of  the  Peruvian  guano,  and  we 
may  take  it  as  a  rule  that  the  richer  they  are  in 
nitrogen  the  more  active  will  that  nitrogen  be.  Meat 
and  fish  guanos,  as  they  are  called,  form  valuable 
fertilisers  for  perennial  crops  like  fruit  and  hops,  and 
may  also  be  mixed  in  small  quantities  with  purely 
mineral  manures  to  form  fertilisers  for  root  crops. 
Rougher  manures  of  this  class  are  sometimes  known  as 
tankage,  or  as  greaves,  and  are  comparatively  slow 
acting,  some  of  them  approach  closely  the  bone  meal 
described  above.  The  nitrogen  they  contain  should  be 
reckoned  as  of  less  value  than  in  the  more  concentrated 
fertilisers.  Naturally  all  these  classes  of  meat  and  fish 
residues  can  only  be  valued  at  the  basis  of  their  analysis. 
A  meat  guano,  for  instance,  may  be  a  highly  nitrogenous 
fertiliser  worth  ;^8  or  £g  a  ton,  or  on  the  other  hand, 
little  better  than  a  bone  meal  costing  half  the  price. 

Very  similar  in  their  actions  to  meat  and  fish 
residues  are  certain  vegetable  residues  which  from  time 
to  time  can  be  purchased  as  fertilisers.  The  best 
known  of  them  is  rape  dust,  which  consists  of  ground- 
up  residues  of  impure  rape  seed  from  which  the  oil  has 
been  extracted  by  chemical  processes.  As  a  rule,  such 
seed   residues  from  which  oil  has  been  extracted  are 


26o  ARTIFICIAL  MANURES  [chap. 

employed  as  cattle  food,  but  sometimes  they  contain 
substances  injurious  to  stock,  like  the  rape  seed  we 
have  just  mentioned  and  the  cake  which  is  derived 
from  pressing  castor-oil  seeds,  or  they  have  become 
damaged  in  some  way  and  are  only  utilisable  as  manure. 
Such  materials  contain,  as  a  rule,  about  5  per  cent,  of 
nitrogen  and  comparatively  small  quantities  of  phos- 
phoric acid  and  potash,  but  when  cheap  they  form  very 
valuable  manures,  especially  on  light  soils.  Many 
other  industrial  residues  get  occasionally  employed  as 
fertilisers,  in  fact,  anything  of  animal  origin,  like  wool 
and  silk,  fur,  hair,  etc.,  contains  nitrogen,  and  is  thereby 
valuable  as  a  fertiliser.  Residues  from  the  textile 
factories  dealing  with  wool  and  silk,  and  fur  or  feathers, 
are  sold  in  the  United  Kingdom  under  the  general  term 
of  shoddies ;  they  are  extremely  variable  in  composition, 
ranging  from  3  to  13  per  cent,  of  nitrogen,  and  they  are 
always  slow  in  their  action,  partly  because  the  material  it- 
self is  not  readily  attacked  by  bacteria,  and  partly  because 
of  the  difficulty  of  getting  the  material  finely  divided 
and  disseminated  through  the  soil.  But  when  they  can 
be  bought  cheaply,  such  residues  form  valuable  fertilisers 
for  perennial  crops.  It  should  be  kept  in  mind  that 
vegetable  fabrics  like  cotton,  linen,  and  jute  contain  no 
nitrogen,  so  that  their  residues  are  valueless  as  manure. 
In  dealing  with  fertilisers  it  is  necessary  that  the 
farmer  should  bear  in  mind  the  very  different  action  of 
the  three  constituents  upon  the  plant,  for  although  all 
three  substances — nitrogen,  phosphoric  acid,  and  potash 
— are  equally  necessary  to  the  growth  of  the  plant,  as 
we  have  seen  when  considering  water  cultures,  yet  they 
possess  very  different  functions  in  its  development. 
Nitrogen  is  mainly  concerned  with  the  vegetative 
development  of  the  plant,  and  increases  the  tendency  to 
form  leaf  and  stem ;  thus  if  a  plant  is  given  an  excess 


XIII.]      SPECIFIC  FUNCTION  OF  FERTILISERS         261 

of  nitrogenous  manure,  the  leaf  system  becomes  exces- 
sive, a  great  number  of  shoots  are  formed,  and  the  plant 
tends  to  go  on  growing  rather  than  to  turn  to  the 
production  of  flowers  and  fruit.  Large  quantities  of 
nitrogen  are  thus  valuable  for  leafy  crops  of  fodder  such 
as  grass,  kale,  cabbages,  etc.  At  the  same  time  it  is 
always  found  that  the  rapid  growth  promoted  by 
excess  of  nitrogen  is  both  soft  and  long-jointed,  and  is 
very  susceptible  to  attacks  of  fungoid  disease.  Cereal 
crops  grown  with  an  excess  of  nitrogen  are  readily  laid 
by  wind  or  rain,  and  the  susceptibility  to  disease  of  plants 
overdosed  with  nitrogen  is  often  well  seen  under  green- 
house conditions.  If  nitrogen  promotes  the  vegetative 
side  of  the  plant,  phosphoric  acid,  on  the  other  hand, 
hastens  maturity  and  favours  the  reproductive  side  of  its 
development,  as,  for  instance,  the  production  of  fruit  and 
seed.  It  is  found  by  experience  that  the  ripening  of 
crops  can  be  accelerated  by  a  liberal  use  of  phosphatic 
manures ;  perennial  plants  like  fruit  trees  can  be 
similarly  induced  to  fruit  rather  than  to  grow.  All 
these  actions  of  phosphoric  acid  are  most  apparent  on 
heavy  soils  and  in  wet  seasons,  when  the  natural  con- 
ditions make  for  slow  maturity.  Phosphatic  manures 
never  give  rise  to  the  immediate  burst  of  growth  and 
the  dark  colour  and  look  of  vigour  which  follows  the 
application  of  nitrogen  ;  their  effect  is  only  to  be  seen  at 
harvest  time,  and  particularly  in  the  proportion  the 
fruit  bears  to  the  rest  of  the  produce.  In  consequence, 
the  use  of  phosphates  is  often  ignored,  while  nitro- 
gen has  been  too  much  employed  because  its  effects 
are  so  manifest.  Potash  is  particularly  concerned  in  the 
manufacture  of  carbohydrates  by  the  plant ;  it  is  there- 
fore particularly  valuable  to  crops  which,  like  mangolds, 
contain  a  good  deal  of  sugar  or,  like  potatoes,  a  good 
deal  of  starch.     Being  so  necessary  to  the  assimilation 


262  ARTIFICIAL  MANURES  [chap. 

by  the  plant,  potash  tends  to  keep  them  growing, 
especially  on  light  soils  and  in  dry  climates,  under  which 
conditions  it  exerts  its  maximum  effects.  It  is  also 
found  to  help  in  stiffening  the  straw  of  cereals  and  grass, 
and  it  increases  the  disease-resisting  powers  of  all  plants. 
But  though  we  can  thus  distinguish  between  the 
effects  of  the  different  fertilising  constituents,  we  are 
rarely  able  to  argue  from  this  knowledge  as  to  the 
requirements  of  the  particular  plants.  That  we  must 
ascertain  by  practical  experiments,  and  as  an  outcome 
of  our  fifty  years'  experience  with  fertilisers,  we  now 
know  pretty  well  the  special  requirements  of  our  various 
farm  crops.  To  some  extent  the  manurial  treatment  of 
a  crop  is  determined  by  the  place  it  occupies  in  the 
rotation  that  is  being  followed.  It  is  comparatively 
rare  to  find  the  same  crop  occupying  the  same  land  year 
after  year,  and  under  ordinary  farming  conditions  the 
soil  very  often  receives  a  fertiliser,  the  effect  of  which  has 
to  extend  over  several  succeeding  crops.  Again,  in  con- 
sidering the  amount  and  nature  of  the  fertilisers  to  be 
purchased,  the  farmer  has  to  take  into  account  the  style 
of  his  farming,  whether  high  or  low.  Wherever  the 
land  is  not  naturally  rich  and  the  markets  are  such  that 
the  farmer  cannot  obtain  a  large  return  per  acre  for  his 
crops,  he  must  cut  his  expenditure  down  to  low  limits, 
and  only  indulge  in  purchased  fertilisers  to  a  very 
limited  extent.  He  must  follow  a  conservative  system 
of  farming,. being  content  with  comparatively  low  yields, 
the  material  for  which  has  in  the  main  been  derived 
from  the  soil.  We  should  always  remember  that  the 
first  application  of  manure  is  the  one  which  produces 
the  largest  increase  in  the  crop,  and  that  if  we  double 
the  manure  bill  we  shall  not  obtain  a  double  yield  or 
even  a  double  increase  over  that  which  is  given  by  one- 
half  the  manure.     In  fact,  if  we  go  on  increasing  the 


XIII.]  FERTILISERS  FOR  WHEAT  263 

amount  of  manure,  we  shall  soon  reach  the  stage  when 
the  last  addition  has  no  effect  at  all.  This  "law  of 
diminishing  returns,"  as  it  is  called,  means  that  a 
farmer  cannot  recoup  himself  for  low  prices  by  forcing 
big  crops  with  the  aid  of  fertilisers.  The  increase  thus 
bought  is  the  dearest  part  of  the  crop.  For  every  farm 
there  is  a  sort  of  level  of  expenditure  on  materials  like 
fertilisers,  and  the  more  profitable  and  the  richer  the 
land,  and  the  more  valuable  the  crops  that  can  be  sold, 
the  higher  will  this  level  become.  Bearing  this  fact  in 
mind,  we  find  that  in  Britain  the  wheat  crop  rarely 
receives  any  fertiliser,  except  perhaps  a  small  top- 
dressing  of  nitrate  of  soda  or  soot  in  the  early  spring. 
Wheat  has  a  long  period  of  growth  and  an  extensive 
root  system,  whereby  it  is  able  to  forage  for  itself  pretty 
thoroughly,  and  obtain  all  the  phosphates  and  potash  it 
requires  from  soil  in  ordinary  good  conditions.  As, 
however,  it  makes  its  growth  during  the  period  of  the 
year  when  the  soil  is  comparatively  cold,  and  as  the  soil 
has  received  very  little  cultivation  before  the  wheat  is 
sown,  and  none  at  all  while  it  is  growing,  the  processes 
which  produce  ammonia  and  nitrates  out  of  the  nitro- 
genous residues  of  the  soil  cannot  be  very  active  at  the 
time  the  wheat  chiefly  requires  its  nitrogen.  Hence  the 
value  of  nitrogenous  fertilisers  to  wheat  in  the  spring, 
whereas  phosphates  and  potash  meet  with  very  little 
response.  If  nitrogen  is  the  dominant  fertiliser  for 
wheat,  barley,  on  the  other  hand,  chiefly  demands 
phosphates.  It  is  a  comparatively  shallow-rooted  plant, 
and  makes  its  growth  in  the  late  spring  on  land  which 
has  been  much  more  thoroughly  prepared  than  that  on 
which  the  wheat  crop  is  sown.  In  the  United  Kingdom 
barley  is  very  often  grown  on  land  which  is  already  com- 
paratively rich  because  the  ground  has  been  previously 
occupied  by  the  turnip  crop,  which  may  even  have  been 


264  ARTIFICIAL  MANURES  [chap. 

consumed  in  situ  by  sheep.  In  such  cases  the  land  is 
already  rich  enough ;  in  fact,  it  contains  too  much  readily 
available  nitrogen  to  give  rise  to  the  best  quality  of 
malting  barley.  But  when  barley  follows  wheat  on 
some  of  the  poor  soils,  a  fertiliser  is  required,  and  this 
should  be  composed  of  a  little  nitrogen  and  a  fair 
amount  of  phosphates.  Oats  can  be  treated  in  much 
the  same  way  as  barley,  but  they  are  often  grown  on 
land  which  has  just  been  broken  up  from  pasture  and 
grass,  in  which  case  they  are  rarely  likely  to  require 
much  fertiliser.  Of  all  the  cereal  crops,  maize  will 
respond  most  freely  to  fertilisers ;  to  it  should  be  given 
all  the  farmyard  manure  that  is  available,  200  or  300 
lb.  per  acre  of  acid  phosphate,  and  perhaps  a  later 
dressing  of  some  active  nitrogenous  manure  like  nitrate 
of  soda.  Of  the  root  crops,  swedes  are  specially 
dependent  upon  phosphoric  acid,  and,  whether  this 
crop  is  grown  by  the  help  of  farmyard  manure  or  by 
artificial  manures  alone,  it  will  be  found  necessary  on 
all  classes  of  land  to  use  for  it  4  or  5  cwt.  to  the  acre  of 
superphosphate  or  basic  slag.  Little  nitrogen  is  wanted, 
because  the  crop  is  growing  in  the  warmer  period  of  the 
year  and  after  a  very  thorough  preparation  of  the  soil, 
so  that  the  production  of  available  nitrogen  compounds 
is  going  on  actively  in  the  soil.  Mangolds,  on  the  other 
hand,  are  more  deeply  rooted  plants,  and  are  sown  at  a 
cooler  time  of  year;  it  is  found  by  experience  that 
they  respond  to  very  considerable  quantities  of  nitrogen, 
and  that  they  also  pay  for  application  of  manures 
containing  potash,  though  phosphatic  manures  are  less 
necessary.  Clovers,  beans,  lucerne  or  alfalfa,  and  other 
leguminous  crops  are  rarely  manured  ;  they  should  not  be 
given  nitrogen,  but  they  will  respond  to  dressings  of 
phosphates  and  potash,  especially  upon  the  lighter  and 
sandier  soils. 


XIII.]  FERTILISERS  FOR  GRASS  LAND  265 

The  manuring  of  grass  land  is  too  big  and 
complicated  a  subject  to  be  dealt  with  here  in  any 
detail.  Very  often  indeed  the  grass  land  receives  no 
artificial  manure  at  all;  the  farmer  trusts  to  the 
fertilising  effects  of  the  cake  and  other  foods  consumed 
by  stock  on  the  land  to  maintain  its  fertility,  even  when 
he  grazes  it  one  year  and  lays  it  up  for  hay  the  next. 
It  will,  however,  be  found  more  profitable  to  hay  the 
same  land  every  year  and  keep  up  its  fertility  by 
manuring  because  in  that  way  the  growth  of  the 
stronger  grasses  which  go  to  make  a  big  hay  crop  is 
encouraged,  whereas  they  are  repressed  by  a  summer's 
grazing  and  their  place  more  or  less  taken  by  smaller 
bottom  herbage  which  cannot  figure  largely  in  the  hay 
crop.  Speaking  generally,  when  supplying  manure  for 
hay  we  want  to  remember  that  nitrogenous  manures 
will  promote  the  growth  of  grasses  at  the  expense  of  the 
other  constituents  of  the  herbage,  until,  as  we  see  on 
some  of  the  Rothamsted  grass  plots,  the  continuance  of 
a  nitrogenous  manure  year  after  year  for  a  long  period 
will  cause  the  whole  herbage  to  be  made  up  of  grass 
alone.  On  the  other  hand,  phosphates  and  potash 
without  nitrogen  stimulate  the  development  of  clovers 
and  other  leguminous  plants.  The  herbage  is  not  so 
bulky,  but  weight  for  weight  is  more  valuable  as  food  for 
stock.  Thus  a  manure  for  hay  should  contain  nitrogen 
in  order  to  get  bulk,  but  should  also  contain  a  due 
proportion  of  phosphates  and  especially  of  potash,  to 
keep  up  the  clovers  and  to  give  feeding  value  to  the 
products.  On  pasture  land  we  want  chiefly  to  encourage 
the  clovers,  and  therefore  use  fertilisers  containing  no 
nitrogen — either  basic  slag  alone  when  the  land  is  rich 
in  potash  which  the  lime  in  the  basic  slag  will  liberate, 
or  basic  slag  and  kainit  on  the  lighter  soils.  If  we  can 
only  encourage  the  growth  of  the  clovers  in  the  pastures 


266  ARTIFICIAL  MANURES  [chap. 

sufficiently,  the  nitrogen  they  collect  from  the  atmo- 
sphere, which  mostly  comes  back  to  the  land  when  the 
herbage  is  grazed  off,  will  be  sufficient  to  keep  up  the 
fertility  of  the  soil. 

As  to  the  time  of  application  of  artificial  manures, 
it  has  already  been  said  that  nitrates  are  not  retained 
by  the  soil,  so  that  nitrate  of  soda  and  sulphate  of 
ammonia,  which  so  readily  changes  into  nitrate,  can  only 
be  employed  as  top-dressings  in  the  spring,  when  the 
crop  already  occupies  the  ground  and  is  ready  to  utilise 
them.  Practically  there  is  very  little  danger  of  even 
the  most  soluble  of  quick-acting  manures  washing  down 
beyond  the  reach  of  the  plant  in  spring  or  summer. 
Consequently  all  kinds  of  fertilisers  may  be  safely 
ploughed  into  the  ground  from  the  month  of  March 
onwards,  except  on  the  very  lighest  soils.  The  applica- 
tion of  spring  manures  is  often  needlessly  delayed  from 
a  mistaken  apprehension  that  materials  like  super- 
phosphate can  be  washed  out  of  the  land.  Insoluble 
fertilisers  like  the  shoddies  amongst  the  nitrogenous 
manures,  and  basic  slag  amongst  the  phosphatic,  should 
be  put  on  as  early  as  possible,  and  may  be  ploughed  or 
dug  into  the  soil  in  the  autumn  or  early  winter. 

Table  XXV.  at  the  end  of  this  chapter  gives  a 
number  of  analyses  of  the  fertilisers  with  which  a  farmer 
is  most  likely  to  meet,  but  it  should  be  borne  in  mind 
that  many  of  these  substances  vary  naturally  in  their  com- 
position, so  that  any  particular  sample  can  only  be 
properly  judged  by  the  analysis  of  that  actual  parcel. 
In  the  United  Kingdom,  and  indeed  in  all  other  civilised 
countries,  the  vendor  must  supply  an  analysis  of  the 
article  he  is  offering  for  sale,  which  analysis  has  the 
force  of  a  guarantee.  Given  such  analyses  of  a  series 
of  suitable  fertilisers,  the  farmer  should  then  learn  to 
value  them  one  against  another.     The  most  ready  way 


XIII.]  VALUATION  OF  MANURES  267 

of  doing  this  is  by  what  is  called  the  unit  system,  the 
unit  being  i  per  cent,  of  a  ton  of  the  fertilising  con- 
stituents— nitrogen,  phosphate  of  lime,  and  potash.  The 
value  of  a  unit  of  each  of  these  constituents  will  vary 
with  market  fluctuations,  and  to  a  certain  extent  with 
the  place  of  delivery.  Thus  the  exact  values  prevailing 
at  any  moment  can  only  be  obtained  by  special  calcula- 
tions, but  for  purposes  of  comparing  one  fertiliser 
against  another  in  the  United  Kingdom  it  will  be 
sufficiently  accurate  to  consider  that  the  unit  of  nitrogen 
is  worth  14s.,  the  unit  of  phosphate  of  lime  2s.  if  it  is 
soluble,  and  I5d.  if  insoluble,  while  the  unit  of  potash 
may  be  reckoned  as  worth  4s.  Working  on  these 
principles,  fish  guano  containing  j\  per  cent,  of  nitrogen 
and  1 3  per  cent,  of  phosphate,  should  be  worth  about 
1 2 IS.,  made  up  as  follows  : — 

^\  of  nitrogen  at  14s. 
+  13  of  phosphate  at  is.  3d. 

Total . 

As  this  fertiliser  was  offered  to  a  farmer  at  146s.,  it  must 
be  regarded  as  dear  when  compared  with  the  meat  meal 
containing  7  per  cent,  of  nitrogen  and  30  per  cent,  of 
phosphate,  which  was  offered  at  the  same  time  at  127s. 
6d.  Valued  on  the  same  principle,  this  later  manure  is 
worth,  7  of  nitrogen  at  14s.  =  98s. +  30  of  phosphate  at 
IS.  3d.  =  37s.  6d.,  or  a  total  of  135s.  6d.  By  making 
valuations  in  this  fashion  of  the  manures  on  offer  in  the 
market,  the  farmer  is  often  able  to  buy  much  more 
cheaply  than  he  would  if  he  stuck  to  the  same  kind  of 
fertiliser  year  by  year,  independently  of  their  fluctua- 
tions in  market  value.  For  some  kinds  of  fertilisers 
which  are  subject  to  natural  variations  it  is  very 
necessary  that  the  farmer  should  obtain  an  analysis 
after  the  delivery  of  the  bulk,  in  order  to  compare  it 


£ 

s. 

D. 

= 

5 

5 

0 

= 

0 

16 

0 

6 

I 

0 

268 


ARTIFICIAL  MANURES 


[chap.  XIII. 


with  the  guarantee  upon  which  he  has  bought.  Such 
analyses  are  rarely  necessary  when  dealing  with 
reputable  merchants  for  such  standard  articles  as  nitrate 
of  soda,  sulphate  of  ammonia,  superphosphates,  and 
potash  salts,  but  all  residual  materials  like  meat  and  fish 
guanos  and  basic  slag  are  subject  to  variations  in  com- 
position which  may  not  have  been  recognised  by  the 
vendor.  When  a  farmer  purchases  large  quantities  of 
any  of  these  articles  it  will  always  pay  him  to  get  the 
deliveries  checked  by  analysis.  However,  as  the 
material  varies  naturally,  it  is  of  the  first  importance  that 
the  farmer  should  sample  the  bulk  very  carefully  so  as  to 
get  a  portion  thoroughly  representative  of  the  whole. 

Table  XXV.— Composition  of  Various  Fertilisers-. 


Phosphoric  Acid 

expressed  as 

Nitrogen. 

Tricalcium  Phosphate. 

Potash. 

Soluble. 

Insoluble. 

Sulphate  of  Ammonia 

20-5 

Nitrate  of  Soda . 

15-7 

... 

... 

... 

Nitrolim  (Cyanamide) 

20«0 

... 

... 

... 

Nitrate  of  Lime 

12.7 

... 

... 

Soot*        .         .         .        . 

3-2 

... 

Wool  Waste*    . 

7.20 

... 

... 

Greaves  *  . 

4-19 

... 

4-75 

Peruvian  Guano  (rich)  *    . 

8.4 

28.7 

2.85 

„            „  (phosphatic)* 

2-37 

47.12 

2.90 

Fish  Meal*        . 

8-68 

... 

22-11 

... 

Meat  Meal  *      . 

6.51 

28.82 

... 

Dried  Blood       . 

9-65 

... 

1.82 

... 

Rape  Dust 

4.84 

... 

3-8 

1.4 

Superphosphate 

26 

2 

n 

37 

1-5 

... 

Basic  Slag 

... 

26 

... 

M                ?>                         • 

... 

48 

... 

Bone  Meal  *      . 

4-50 

... 

46-92 

... 

Dissolved  Bones  * 

3-21 

1 1 '64 

26-69 

... 

Kainit 

... 

... 

12-8 

Subject  to  considerable  variation. 


CHAPTER  XIV 

MILK,   BUTTER,  AND   CHEESE 

Composition  of  Milk.  Variations  to  which  the  Composition  of 
Milk  is  subject.  Effect  of  Individuality,  Breed,  Food,  Time  of 
Milking,  Period  of  Lactation.  Feeding  for  Milk.  Composi- 
tion of  Butter.  Nature  of  the  Churning  Process.  Effect  of 
various  Foods  upon  the  Quality  of  the  Butter.  Composition 
of  Cheese.  Changes  taking  place  during  the  Cheese-making 
Process.  The  Ripening  of  Cheese.  Importance  of  Cleanli- 
ness in  all  dealings  with  Milk. 

Milk  is  not  a  simple  substance,  but  a  mixture  of  a 
large  number  of  different  bodies  which  are  present  in 
fairly  definite  quantities,  though  they  are ,  subject  to 
certain  variations  to  be  discussed  later.  By  weighing 
out  a  small  quantity  of  milk  into  a  dish  so  that  the  milk 
only  forms  a  thin  layer  over  the  bottom,  and  putting 
the  whole  into  an  oven  to  dry,  we  can  show  that  the 
greater  part  of  milk  consists  of  water,  there  being,  as 
a  rule,  not  more  than  1 2  per  cent  of  total  solids  in  the 
milk.  This  important  figure — the  amount  of  total 
solids  in  the  milk — can  only  be  determined  exactly  by 
getting  the  milk  spread  out  into  a  very  thin  layer  before 
drying  it,  so  readily  does  a  skin  form  over  the  surface 
and  cause  the  drying  of  the  rest  of  the  milk  to  proceed 
with  difficulty.  Among  the  milk  solids  at  least  three 
are  characteristic.  In  the  first  place  there  is  the  fat, 
present  to  the  extent  of  3  per  cent,  or  more;  then 
proteins  to  the  extent  of  about  3  J  per  cent,  the  greater 
part  of  which  is  commonly  called  casein  ;  then  lactose  or 


270  MILK,  BUTTER,  AND  CHEESE  [chap. 

milk  sugar  forms  another  4J  per  cent.  In  addition  to 
these  three  carbon  compounds,  the  milk  solids  contain 
certain  inorganic  materials,  chiefly  phosphates  and 
chlorides  of  calcium  and  potassium  and  sodium,  some 
of  which  inorganic  materials  are,  however,  combined 
with  the  proteins. 

Milk  Fat. — The  fat  of  milk  is  there  present  in  a 
state  of  finely  divided  globules  varying  in  size  from 
o-oi  mm.  to  o-ooi  mm.,  i.e,  from  25^60  of  an  inch  to  yV 
of  that  diameter.  When  examined  under  the  micro- 
scope these  minute  globules  appear  to  have  a  skin  upon 
the  surface,  but  this  skin  probably  consists  of  nothing 
more  than  adhering  particles  of  the  casein,  which  also 
is  not  truly  dissolved  in  the  water  of  the  milk.  The  fat 
globules  vary  in  size,  the  milk  of  Jersey  cows  contain 
the  largest,  in  the  Guernseys  and  in  the  other  Channel 
Island  breeds  they  are  almost  as  large,  and  they  are 
also  well  above  the  average  size  in  the  milk  of  Kerrys. 
In  the  milk  of  the  Shorthorns,  globules  of  various  sizes 
may  be  found,  but  the  small  predominate ;  the  Welsh 
and  South  Devon,  the  Dutch  and  Holstein  races, 
yield  milk  containing  rather  small  globules.  In  these 
globules  the  fat  is  present  in  the  liquid  state,  although 
the  temperature  of  the  milk  may  have  cooled  down 
below  the  temperature  at  which  the  fat  solidifies  in 
bulk.  In  such  small  particles,  however,  the  fat  remains 
liquid  in  a  supercooled  state,  only  assuming  a  solid 
condition  when  the  globules  are  beaten  together,  as  in 
the  act  of  churning.  The  fat  is  lighter  than  water, 
possessing  a  specific  gravity  of  0-93.  In  consequence, 
the  globules  tend  to  rise  upwards  through  the  heavier 
milk  serum,  and  they  are  only  hindered  from  rising 
rapidly  by  their  smallness  and  the  high  viscosity  of  the 
milk  serum.  The  larger  the  particles,  however,  the 
more  quickly  do  they  rise,  as  may  be  seen  in  the  quicker 


XIV.]        THE  COMPOSITION  OF  BUTTER  FAT         271 

and  more  thorough  creaming  of  the  milk  from  Jersey 
cows.  When  the  milk  fat  is  separated  and  examined  it 
proves  to  be  a  complicated  substance,  consisting,  like  all 
fats,  of  a  combination  of  glycerin  with  certain  so-called 
fatty  acids.  In  butter  fats  there  are  a  large  number  of 
these  fatty  acids  present,  differing  from  one  another  in 
their  physical  constituents,  some  members  being  volatile 
and  strongly  smelling  liquids,  while  others  are  solid 
substances,  possessing  little  smell  or  taste.  We  can 
obtain  an  idea  of  these  acids  of  butter  fat  by  taking 
about  a  gramme  of  the  clarified  fat  and  warming  it  with 
20  C.C.  of  a  solution  of  potash  in  alcohol.  If  the  fat  is 
stirred  up  it  will  eventually  dissolve,  the  glycerin  being 
set  free,  while  the  acids  combine  with  the  potash  to  form 
a  soap.  The  mixture  is  now  dried  up  to  get  rid  of  the 
alcohol,  and  treated  with  dilute  sulphuric  acid,  which 
decomposes  the  soap  and  sets  free  the  fatty  acids. 
These  substances  will  then  be  found  to  possess  a  very 
powerful  smell  of  rancid  butter,  the  smell  being  due  to 
butyric  and  one  or  two  of  the  other  volatile  acids.  As 
the  liquid  cools  the  non-volatile  acids,  which  are  also 
insoluble  in  water,  will  solidify  to  a  cake  on  the  surface 
of  the  liquid.  The  flavour  of  butter  is  due  to  the 
liberation  of  a  very  small  trace  of  some  of  these  volatile 
fatty  acids  through  the  action  of  bacteria  upon  the 
butter  fat.  If  too  much  is  liberated  the  smell  and  taste 
become  too  pronounced,  and  we  call  the  butter  rancid. 
The  composition  of  the  butter  fat  itself  is  not  constant, 
but  varies  with  the  season  of  the  year,  the  nature  of  the 
food,  and  the  period  of  lactation.  Hence  follows  a 
variability  in  the  composition  of  pure  butter  fat  which 
constitutes  the  chief  difficulty  in  detecting  adulterations 
of  butter,  for  most  of  the  methods  of  analysing  butter 
depend  upon  determinations  of  the  proportion  of  volatile 
and  insoluble  fatty  acids. 


272  MILK,  BUTTER,  AND  CHEESE  [chap. 

Protein. — Of  the  proteins  of  milk  the  chief  is  a  body 
called  casein,  which  is  distinguished  by  its  property  of 
forming  a  firm  curd  when  treated  with  the  enzyme  of 
rennet,  or  a  flocculent  curd  when  treated  with  acids. 
The  casein  is  a  compound  of  carbon  containing  15-7  per 
cent,  of  nitrogen  and  about  -8  per  cent,  of  sulphur  and 
phosphorus  respectively.  It  is  present  in  milk 
probably  combined  with  calcium  ;  the  resulting  salt  is 
not  soluble  in  water,  but  exists  in  normal  milk  in  a 
finely  divided  state  diffused  through  the  serum,  forming 
what  is  known  as  a  colloidal  suspension.  Rennet, 
which  consists  of  a  solution  of  the  enzyme  which  is 
present  in  the  fourth  stomach  of  a  calf,  causes  the 
casein  to  coagulate  and  form  a  firm  curd,  in  which  are 
also  enclosed  the  fat  globules  of  the  milk.  The  rapidity 
of  the  action  of  the  rennet  and  the  firmness  of  the 
resulting  curd  are  increased  at  higher  temperatures,  but 
the  presence  of  a  soluble  lime  salt  is  necessary  before 
the  curd  will  form.  For  this  reason,  perfectly  fresh 
milk  will  not  curdle  with  rennet,  a  certain  amount  of 
acid  must  first  have  been  developed,  which  formation  of 
acid  takes  place  naturally  when  the  milk  stands,  by  the 
action  of  certain  bacteria,  which  always  find  their  way 
into  the  milk  and  convert  the  milk  sugar  into  lactic 
acid,  the  substance  characterising  sour  milk.  It  will  be 
seen  later  that  the  development  of  acidity  and  the 
establishment  of  a  suitable  temperature  are  very 
essential  to  obtain  the  curd  of  proper  consistency  for 
cheese-making.  After  the  casein  has  been  precipitated 
from  milk  by  the  action  of  rennet  or  acids,  the  milk  still 
contains  about  o-6  per  cent,  of  other  proteins,  chiefly  of 
albumen,  which  begins  to  coagulate  when  the  whey  is 
heated  to  temperatures  above  70°. 

Milk   Sugar. — The  solid   body  which  is  present  in 
largest  amount  in  milk  is  a  particular  sugar  known  as 


XIV.]  MILK  SUGAR  273 

lactose,  which  differs  from  ordinary  sugar  in  its  lesser 
solubility  and  in  its  lack  of  sweetness.  Milk  sugar  may 
be  obtained  by  evaporating  down  the  whey  after  the 
casein  has  been  coagulated  and  the  albumen 
removed  by  heating  up  to  boiling-point  Milk  sugar 
does  not  ferment  with  ordinary  yeast, '  but  can  be 
fermented  into  alcohol  and  carbon  dioxide  by  certain 
special  yeasts  such  as  those  employed  in  making  kephir 
and  koumiss — fermented  liquids  used  by  the  Mongol 
races.  The  most  characteristic  fermentation  of  milk 
sugar,  however,  is  that  brought  about  by  the  lactic  acid 
bacteria,  which  are  always  about  in  cows'  stalls,  etc.,  and 
find  their  way  into  the  milk ;  they  multiply  with  great 
rapidity  in  the  warm  milk,  and  split  up  the  sugar  into 
lactic  acid,  while  at  the  same  time  they  oxidise  some  of 
it  into  carbon  dioxide  and  water.  These  organisms  are 
responsible  for  the  usual  natural  souring  of  milk,  in 
which  the  casein  is  precipitated  as  soon  as  the  acidity 
reaches  a  certain  degree.  It  is  because  the  development 
of  the  lactic  acid  .bacteria  is  so  much  accelerated  by 
warmth,  that  it  is  important  to  cool  milk  down  by  the 
refrigerator  immediately  after  it  has  been  drawn  from 
the  cow,  if  the  milk  is  to  be  sent  any  distance  by  rail 
and  not  used  immediately. 

Composition  of  Milk. — The  average  composition  of 
milk  in  England  is  given  by  Droop  Richmond  as 
follows,  the  figures  being  the  average  of  about  200,000 
analyses : — 


Fat 

3-90 

Protein  . 

3-50 

Lactose 

475 

Ash 

075 

Water    . 

87-10 

Total       . 

I  GO-GO 

274  MILK,  BUTTER,  AND  CHEESE  [chap. 

Though  this  average  composition  is  very  well  main- 
tained when  a  large  number  of  analyses  are  considered, 
the  milk  from  a  given  cow  may  vary  very  widely  from 
the  average,  and  these  variations  are  governed  by  the 
factors  set  out  below.  It  will,  however,  be  found  that 
the  chief  variable  is  the  amount  of  fat;  though  the 
solids  also  vary,  the  differences  are  never  so  great  as 
those  of  the  fat. 

1.  Individuality. — In  the  milk  of  the  cows  compos- 
ing any  given  herd,  although  they  may  be  all  animals 
of  the  same  age  belonging  to  the  same  breed  and  all 
treated  alike  as  regards  food  and  housing,  there 
will  be  found  considerable  variations  in  composi- 
tion. In  an  ordinary  mixed  herd  of  Shorthorns, 
cattle  not  specially  selected,  it  will  generally  be  found 
that  some  of  the  cows  habitually  yield  milk  containing 
less  than  3  per  cent,  of  fat,  while  in  others  the  percent- 
age is  well  over  4.  These  differences  are  known  to  be 
hereditary,  so  that  not  only  can  the  average  composition 
of  the  mixed  milk  of  the  herd  be  considerably  raised  by 
weeding  out  the  animals  yielding  poor  milk,  but  by 
steadily  breeding  only  from  those  cows  which  yield 
milk  possessing  a  high  percentage  of  fat,  a  very  consider- 
able and  permanent  improvement  may  be  effected  in 
the  milk. 

2.  Breed. — It  has  already  been  mentioned  that  the 
Jerseys  and  other  Channel  Island  cattle  yield  milk  in 
which  the  fat  globules  are  above  the  average  size.  It  is 
also  found  that  these  races  produce  milk  containing  a 
higher  proportion  of  fat.  The  following  table  shows  the 
average  analyses  of  the  milk  of  a  number  of  races  of 
cows  which  are  usually  kept  in  milk  in  the  British  Isles. 
The  effect  of  individuality  must  also  be  superimposed 
upon  these  average  figures,  so  that  single,  cases  occur  in 


XIV.] 


VARIATIONS  OF  FAT  IN  MILK 


275 


which  the  butter  fat  in  the  milk  of  Jerseys  rises  to  6  or 
7,  and  even  10,  per  cent. 

Table  XXVI.— Percentage  of  Fat  in  Milk  of 
Different  Breeds. 


Veith, 

New  Jersey, 

R.A.8.E.  Show, 

England. 

U.S.A. 

1909. 

Jersey  .... 

5.66 

4.78 

4.29 

Guernsey 

5-02 

4-47 

Kerry  .... 

4.72 

... 

4-13 

Red  Poll       . 

4-34 

.. 

2.99 

Shorthorn     . 

403 

3-65 

3-07 

A3rrshire 
Holstein 

3-68 

3-79 

3-51 

3.  Food. — It  is  commonly  supposed  that  the  per- 
centage of  fat  in  the  milk  of  a  given  herd  can  be  raised 
by  feeding  concentrated  foods  rich  in  fat.  Experiments, 
however,  show  that  the  difference  which  can  be  brought 
about  in  this  way  is  very  small ;  the  proportion  of  fat 
contained  in  the  milk  yielded  by  a  given  cow  being  a 
physiological  function  of  the  cow  itself,  it  is  compara- 
tively unaffected  by  the  food.  If  the  amount  of  food 
given  be  insufficient  for  the  average  requirements  of  the 
cow,  the  animal  will  begin  to  lose  weight,  and  will  draw 
the  material  from  its  own  tissues  in  order  to  keep  the 
yield  of  milk  and  the  proportion  of  butter  fat  up  to  its 
normal  percentage;  only  when  the  cow  has  lost  an 
appreciable  amount  of  weight  will  both  the  yield  and 
the  richness  of  the  milk  begin  to  fall  off  more  markedly. 
On  the  other  hand,  an  excess  of  rich  food  will  cause  the 
cow  to  put  on  fat,  and  as  the  cow  gets  very  fat  the  milk 
will  again  begin  to  fall  off  both  in  quality  and  quantity. 
A  high  proportion  of  fat  in  the  food  does  not  make  the 
milk  any  richer  in  fat.  The  chief  effect  of  a  liberal  but 
not  excessive  diet  appears  to  be  to  maintain  the  milk 
yield  somewhat  longer  during  the  period  of  lactation 


276  MILK,  BUTTER,  AND  CHEESE  [chap. 

than  would  otherwise  be  the  case.  Nor  do  particular 
foods  have  any  permanent  effect  upon  the  richness  of 
the  milk.  It  is  generally  found  that  a  slight  change  of 
food  exerts  a  stimulating  action  for  a  short  time  upon 
the  milk  yield,  though  drastic  changes  may  have  the 
reverse  effect  and  temporarily  reduce  the  yield  and 
quality  of  the  milk  ;  in  any  case  the  effect  does  not 
persist  for  many  days.  Certain  classes  of  succulent 
foods  which  are  very  rich  in  ^-proteins,  such  as  brewer's 
grains,  green  fodder,  and  especially  fresh  grass  at  its 
first  shoot  in  the  spring,  have  an  exciting  effect  in 
promoting  the  flow  of  milk.  Particularly  does  the  first 
grass,  after  the  cows  are  turned  out,  ^\mq  rise  to  an 
abundant  production  of  comparatively  poor  milk.  Lack 
of  green  food  is  also  apt  to  result  in  a  falling-off  both  in 
the  yield  and  the  quality  of  the  milk,  and  a  large 
quantity  of  watery  foods  eventually  results  in  thin  and 
poor  milk. 

The  quality  of  the  butter  is,  however,  considerably 
affected  by  the  nature  of  the  food,  because  the  composi- 
tion of  the  butter  fat  varies  with  the  material  out  of 
which  it  is  manufactured  by  the  cow;  in  some  cases 
indeed  the  fatty  acids  contained  in  the  food  can  again 
be  identified  in  the  butter  fat.  An  excess  of  fibrous 
foods — hay  and  straw  or  over-ripe  forage  crops — gives 
rise  to  hard,  tasteless  butter ;  peas  and  beans  and  cotton- 
seed meal  or  cake  also  harden  the  butter.  On  the  other 
hand,  linseed  cake  gives  rise  to  a  soft  and  oily  butter, 
and  maize  and  gluten  feeds,  oats  and  rice,  also  tend  to 
soften  the  butter.  The  feeding  of  turnips  and  swedes, 
except  some  time  before  milking,  communicates  a 
characteristic  and  disagreeable  flavour  to  the  butter, 
while  it  is  well  known  that  the  flavour  of  certain  strongly 
smelling  plants  like  wild  garlic  is  carried  over  to  the 
butter. 


XIV  ]  VARIATIONS  OF  FAT  IN  MILK  277 

4.  Period  of  Lactation. — Immediately  after  calving 
the  cow  yields  milk  of  abnormal  composition,  the 
product  being  a  thick  yellowish  liquid,  which  coagulates 
on  boiling  and  is  known  as  colostrum  or  beastings. 
The  colostrum  at  first  contains  only  about  70  per  cent, 
of  water  and  something  like  20  per  cent,  of  proteins,  of 
which  albumen  is  by  far  the  most  abundant,  the  casein 
being  present  in  only  about  the  normal  proportion.  After 
four  or  five  days  the  colostrum,  which  is  necessary  for 
the  first  nutrition  of  the  calf,  passes  into  ordinary  milk. 
During  the  first  month  or  two  after  calving  the  yield  of 
milk  is  at  its  greatest,  and  then  gradually  falls  off  until 
the  cow  becomes  dry,  after  nine  or  ten  months.  During 
the  first  flush  of  milk  the  proportion  of  butter  fat  is  at 
its  lowest,  it  then  reaches  its  average  amount,  and  after- 
wards rises  again  as  the  cow  begins  to  dry  off.  The 
influence  of  the  period  of  lactation  may  be  to  some 
extent  disguised  by  the  time  of  year  at  which  the  cow 
calves  down,  and  the  effect  of  such  changes  as  turning 
the  cow  out  to  grass. 

5.  Age, — While  it  is  difficult  to  obtain  strictly  com- 
parable results,  there  is  evidence  that  the  milk  yield 
improves  up  to  about  eight  years  of  age,  after  which  a 
decided  falling-off  begins  to  set  in  about  the  twelfth 
year.  The  percentage  of  butter  fat  drops  a  little  for  the 
first  few  years,  and  then  remains  constant  until  the 
eleventh  year  or  so,  after  which  it  begins  to  fall 
rapidly. 

6.  Time  of  Milking. — As  a  rule,  in  Great  Britain 
cows  are  milked  twice  a  day,  at  intervals  separated  by 
very  unequal  lengths  of  time.  It  is  not  uncommon,  for 
example,  to  find  cows  milked  at  five  or  six  o'clock  in 
the  morning  and  again  about  two  o'clock  in  the  after- 
noon, thus  dividing  the  day  into  one  period  of  sixteen 
and  another  of  eight  hours.     It  is  always  found  that 


278 


MILK,  BUTTER,  AND  CHEESE 


[chap. 


after  a  long  interval,  i.e.,  generally  in  the  morning  milk, 
the  yield  is  larger,  but  there  is  a  corresponding  falling- 
ofif  of  quality.  The  morning's  milk  very  often  contains 
less  than  3  per  cent,  of  fat,  when  the  evening's  milk  may 
contain  well  over  4,  and  the  average  milk  of  the  herd  is 
something  like  3J  per  cent.      The  following  table  will 

Table  XXVII. — Variation  of  Yield  and  Composition  of 
Milk  with  Interval  of  Milking. 


Morning  Milk. 

Evening  Milk. 

Yield. 

Per  cent. 
Fat. 

Yield. 

Per  cent. 
Fat. 

Thirteen  and  eleven  hours     . 
Fifteen  and  nine  hours . 

1-2 

1-5 

3-18 
2-87 

I 
I 

3-8 
4.26 

Equal  intervals     .... 
Sixteen  and  eight  hours 

1-05 
1-5 

3-64 
2-33 

I 
I 

3-45 
4-47 

show  an  example  in  these  variations  in  yield  and  com- 
position. It  is  impossible  to  get  over  these  variations 
by  any  alteration  in  the  food  of  the  animals,  although, 
as  a  consequence,  the  morning's  milk  often  falls  below 
the  legal  standard  of  butter  fat.  In  order  to  avoid 
trouble  on  this  score  it  is  necessary  to  milk  at  intervals 
of  twelve  hours,  and  hold  back  the  evening's  milk  in  a 
refrigerated  condition  for  the  morning  delivery.  Milking 
three  times  a  day  is  sometimes  resorted  to,  and  there  is 
some  evidence  that  the  secretion  of  milk  is  thereby 
stimulated. 

The  milk  that  is  first  drawn  from  the  udder  is  always 
poorer  than  the  last  drawn,  or  the  strippings.  Conse- 
quently, the  milker  can  seriously  lower  the  average 
composition  of  the  milk  unless  care  is  taken  to 
thoroughly  strip  the  cow  and  make  her  yield  all  the 


XIV.] 


VARIATIONS  OF  FAT  IN  MILK 


279 


milk  that  is  possible.  Table  XXVIII.  shows  the  amount 
of  butter  fat  in  the  successive  portions  of  milk  taken 
from  the  cow. 

Table  XXVIII.— Composition  of  Successive  Portions  of 
Milk  taken  from  the  Cow. 


Total  solids     . 
Fat .        . 

10-47 
1.70 

IC'75 
1.76 

10-85 
2-10 

11-23 

2-54 

11-63 

3-14 

12-67 
4-08 

7.  Other  causes  of  disturbance. — It  is  found  that 
cows  are  extremely  susceptible  animals,  easily  disturbed 
both  in  the  yield  and  composition  of  their  milk  by  any 
external  causes  which  excite  the  animals.  Change  of 
location  into  a  new  building,  going  out  to  grass,  any 
sudden  fright,  a  thunderstorm,  or  a  marked  change  of 
temperature,  will  often  be  found  to  exert  a  considerable 
influence  upon  both  the  yield  and  the  composition  of 
milk.  In  fact,  if  the  composition  of  the  milk  of  a  single 
cow  be  examined  day  by  day,  it  will  be  found  to  show 
a  number  of  irregular  variations  both  in  amount  and 
composition,  which  are  to  be  accounted  for  by  minor 
disturbances  of  this  kind.  It  is  thus  necessary,  when 
one  wishes  to  ascertain  the  average  composition  of  the 
milk  produced  by  a  given  cow,  to  make  up  a  composite 
sample  representing  the  milk  yielded  during  at  least 
one  week.  This  should  be  done  by  putting  aside  after 
each  milking  a  small  quantity  proportional  to  the  yield 
on  that  occasion,  a  preservative  being  added  to  the 
mixed  sample  in  order  to  retain  it  in  a  condition  for 
analysis  up  to  the  end  of  the  week. 

From  these  particulars  it  will  be  seen  that-  the  farmer 
engaged  in  the  production  of  milk  should  first  of  all  pay 
attention  to  the  yield  and  composition  of  the  milk 
produced    by   each   individual   cow.      By   keeping   his 


28o  MILK,  BUTTER,  AND  CHEESE  [chap. 

records  of  the  milk  yield  of  each  cow  from  day  to  day, 
and  by  occasional  analyses,  he  will  be  enabled  to  obtain 
the  most  profitable  returns  from  his  cows.  Cases  are 
on  record  where  the  milk  yield  of  a  herd  has  been 
raised  by  lOO  or  200  gallons  per  cow  per  year  by  the 
keeping  of  careful  records  and  the  weeding  out  of  the 
poorer  animals.  If  the  farmer  is  selling  milk  it  will  be 
sufficient  for  him  to  obtain  occasional  analyses  of  the 
mixed  milk  in  order  to  see  that  it  keeps  above  the  legal 
standard  ;  but  if  he  finds  it  is  falling  too  low,  it  may  be 
necessary  to  examine  into  the  composition  of  the  milk 
of  the  single  cows,  in  order  to  weed  out  those  who  are 
reducing  the  percentage  of  fat  to  a  dangerous  degree. 
When,  however,  a  farmer  is  making  butter  from  his 
milk  he  is  less  concerned  with  the  total  amount  of  milk 
yielded  than  with  the  amount  of  butter  fat  itself,  and  it 
is  to  such  a  farmer  that  a  knowledge  of  the  average 
composition  of  the  milk  yielded  by  each  animal  is  of  the 
first  importance.  As  regards  the  feeding,  we  have  seen 
that  this  should  be  liberal  but  not  excessive,  if  either  too 
high  or  too  low  the  cow  will  begin  to  fall  off  in  both  yield 
and  quality  after  a  certain  time.  Perhaps  the  best  test 
that  the  food  is  being  maintained  at  a  right  level,  is  to 
weigh  the  cows  from  time  to  time.  If  they  are  slightly 
gaining  in  weight  we  may  be  sure  that  the  food  is  at 
about  the  right  level.  Very  often  in  this  country  cows 
are  fed  excessively  and  wastefully  owing  to  the  mistaken 
idea  that  the  greater  and  richer  the  amount  of  food  the 
better  will  be  the  production  of  milk.  Perhaps  the 
greatest  economy  in  feeding  cows  can  be  effected  by 
roughly  adjusting  the  amount  of  food  to  the  weight  of 
the  cow  and  the  yield  of  milk.  Properly  speaking,  a 
cow's  rations  should  be  made  up  of  the  amount  of  food 
required  for  maintenance,  which  will  vary  simply 
with    the    weight    of    the    cow,    together    with    the 


XIV.]  FOOD  REQUIRED  BY  COWS  281 

amount  required  for  the  production  of  the  milk  she 
yields,  which  may  be  taken  to  bear  approximately  the 
same  relation  to  the  food  as  the  live  weight  increase 
does  in  the  case  of  the  fattening  animals.  For  practical 
purposes  considerable  economy  can  be  effected  in  the 
feeding  of  the  herd  if  the  cows  are  divided  into  three  or 
four  groups,  according  to  their  weight  and  their  milk 
yield.  The  cows  should  have  first  a  basal  ration,  supply- 
ing each  with  about  25  lb.  of  dry  food  per  1000  lb. 
live  weight,  this  food  to  contain  about  i  lb.  of  digestible 
protein  and  J  lb.  digestible  fat.  Next,  for  each  10  lb.  of 
milk,  food  with  a  starch  equivalent  of  about  5  J  lb.  and 
containing  about  \\  lb.  of  protein,  is  necessary.  The 
cows  can  easily  be  divided  into  three  groups  according 
to  their  milk  yields  (allowing  also  a  little  for  their  live 
weight),  and  given  one,  two,  or  three  measures  of  con- 
centrated food  in  addition  to  the  basal  ration  which  all 
receive  alike. 

Butter 

The  essential  feature  of  butter-making  consists  in 
agitating  either  the  whole  milk  or  the  cream  until  the 
fat  globules  coalesce  and  form  clots  the  size  of  shot,  at 
which  stage  the  butter  is  said  to  have  "  come."  Whole 
milk  is  very  rarely  churned  nowadays;  instead,  the  milk 
is  first  of  all  set  for  twenty-four  or  forty-eight  hours  and 
the  cream  skimmed  off,  or  the  cream  is  separated  by 
some  mechanical  separator,  as  in  all  modern  dairies. 
In  the  old-fashioned  methods  of  making  butter,  the 
milk,  when  still  warm,  is  poured  into  shallow  pans,  which 
are  left  to  stand  in  a  cool  dairy,  whereupon  the  fat 
globules  slowly  rise  to  the  surface,  and  the  layer  of 
cream  is  skimmed  off.  The  separation,  when  effected 
in  this  way,  is  not  very  perfect,  about  0-8  per  cent,  of  the 
fat  being  left  in  the  skimmed  milk.     Instead  of  shallow 


282  MILK,  BUTTER,  AND  CHEESE  [chap. 

pans,  deep  metal  vessels  jacketed  with  ice  were  some- 
times employed,  by  which  means  a  very  effective 
separation  of  cream  was  brought  about,  but  an  even 
larger  quantity  of  fat,  over  i  per  cent,  was  left  in  the 
milk.  The  only  real  gain  in  this  process  was  that  the 
cream  was  protected  from  the  influx  of  the  harmful 
bacteria.  Where  any  large  quantity  of  butter  is  made, 
the  mechanical  separator  is  now  invariably  used. 
These  instruments  depend  for  their  action  upon 
bringing  the  milk  into  a  violent  whirling  motion  by 
making  it  flow  into  a  bowl  revolving  at  the  rate  of 
about  6000  revolutions  per  minute.  Under  these 
conditions  the  force  of  gravity,  which  tends  to  separate 
the  lighter  fat  globules  from  the  denser  milk  serum,  is 
exchanged  for  a  centrifugal  force  many  times  greater, 
which  draws  the  heavier  serum  to  the  outside  of  the 
bowl  and  leaves  the  lighter  cream  at  the  centre,  suitable 
ducts  being  arranged  to  take  away  the  two  liquids  which 
have  thus  been  separated.  The  cream  which  is  obtained 
from  the  separator  may  contain  anything  from  15  to  50 
per  cent,  of  fat,  according  to  the  velocity  at  which  the 
separator  is  run.  The  separated  milk,  which  is  the 
name  given  to  the  liquid  from  which  the  cream  has 
been  removed,  contains  as  little  as  o- 1 5  per  cent,  of  fat 
when  the  separator  is  working  properly ;  the  rest  of  the 
serum  possesses  the  same  composition  as  it  does  in 
uncreamed  milk.  After  the  cream  has  been  separated 
it  may  be  churned  at  once,  in  which  case  the  product 
is  known  as  sweet-cream  butter,  but  the  flavour  is 
then  somewhat  inferior  and  the  yield  of  butter  is  lower. 
It  is  customary  to  put  aside  the  cream  for  at  least  forty- 
eight  hours  in  order  to  develop  a  certain  amount  of 
acidity  before  churning.  While  the  cream  is  thus 
ripening  it  should  stand  in  a  warm  place,  as  nearly  as 
possible  65°  F.  and  should  be  stirred  from  time  to  time 


XIV.]  CHURNING  283 

in  order  to  aerate  it.  Unless  the  cream  is  kept  aerated 
the  bacteria  making  lactic  acid  may  give  place  to  others 
producing  butyric  acid,  to  the  great  detriment  of  the 
flavour  of  the  resulting  butter.  When  the  cream  is 
properly  ripe  it  should  be  brought  to  an  appropriate 
temperature  before  churning,  this  temperature  being 
about  60°  in  the  vi^inter  and  55°  in  the  summer.  The 
length  of  time  occupied  by  churning  will  depend  upon 
getting  a  proper  proportion  of  cream  and  water  and  a 
correct  adjustment  of  temperature.  When  the  churning 
begins,  it  is  necessary  with  sour  milk  in  a  closed  churn 
to  open  the  ventilator  in  the  churn  from  time  to  time 
during  the  first  five  minutes  of  churning.  The  agitation 
of  the  cream  liberates  a  certain  amount  of  carbon 
dioxide  which  had  been  formed  by  the  lactic  acid 
bacteria  and  was  dissolved  in  a  supersaturated  condition 
in  the  serum  of  the  cream,  and  unless  this  carbon 
dioxide  is  liberated  from  the  churn  the  whole  of  the 
cream  will  pass  into  a  frothy  or  whipped  condition.  As 
soon  as  the  granules  of  butter  have  reached  the  size  of 
small  shot  the  process  is  nowadays  stopped,  and  the 
butter  is  washed  to  free  it  from  all  adhering  milk  serum 
before  it  is  worked  up  into  pats.  Unless  this  washing 
is  thorough  the  butter  will  not  keep,  because  the  small 
amount  of  casein  and  milk  sugar  left  in  when  the 
washing  is  imperfect  provides  a  very  favourable  nutrient 
medium  for  the  development  of  bacteria,  which  will  split 
up  the  butter  fat  and  set  free  some  of  the  disagreeably 
flavoured  fatty  acids.  The  resulting  butter  is  not  pure 
butter  fat  but  contains  a  proportion  of  water  depending 
on  the  velocity  and  the  temperature  of  the  churning, 
and  also  upon  the  method  of  working  the  butter  after- 
wards. Rapid  churning  at  high  or  at  very  low  tempera- 
tures will  give  rise  to  a  number  of  very  finely  divided 
globules  of  water  inside  the  butter  fat,  and  these  cannot 


284  MILK,  BUTTER,  AND  CHEESE  [chap. 

be  removed  by  any  subsequent  working.  What  visible 
water  there  is  in  butter  has  been  left  in  through 
imperfect  work  on  the  butter  table.  Well-made  butter 
should  not  contain  more  than  15  per  cent,  of  water, 
though  as  much  as  18  may  be  not  unfrequently  found. 
In  addition  to  the  water  a  certain  amount  of  curd,  i.e., 
coagulated  casein,  and  of  lactose  derived  from  the  cream 
serum,  are  always  present,  together  with  whatever  salt 
has  been  added  to  the  butter  in  the  process  of  making 
up.  From  this  account  it  will  be  seen  that  no  chemical 
action  takes  place  during  the  making  of  butter ;  it  is  a 
mechanical  process  whereby  the  fat  globules  are  made 
to  coalesce  by  being  beaten  together  until  the  surface 
layer  which  prevents  them  from  uniting  in  milk  has 
been  broken  down.  Success  in  churning  depends  upon 
obtaining  the  right  degree  of  acidity  at  starting,  and  then 
working  at  the  proper  temperature.  In  the  further 
treatment  of  butter,  cleanliness  is  the  chief  essential. 

Cheese 

The  process  of  cheese-making  is  much  more  elabor- 
ate than  that  of  making  butter.  Moreover,  from  the 
very  large  number  of  different  kinds  of  cheese  whose 
characteristics  are  developed  by  special  methods  of 
making  and  curing,  it  would  be  beyond  the  scope  of 
this  book  to  discuss  the  working  details  of  the  processes 
which  result  in  any  particular  cheese ;  instead,  it  will  be 
sufficient  to  consider  very  broadly  the  principles  involved 
in  the  making  of  one  variety,  Cheddar,  because  these 
principles  apply  to  all  other  cheeses.  Cheese-making 
may  start  with  either  whole  milk,  separated  milk,  or 
milk  from  which  a  portion  of  the  cream  has  been 
removed ;  but  it  is  desirable  that  the  milk  should  be 
derived  from  Ayrshires,  Shorthorns,  or  Dutch  cows, 
which  do  not  give  rise  to  large  globules  of  fat  readily 


XIV.]  CHEESE-MAKING  285 

separating  from  the  serum.  It  is  necessary  to  begin 
with  a  certain  degree  of  acidity  in  the  milk.  This  is 
obtained  by  leaving  the  vats  to  stand  at  a  proper 
temperature,  stirring  from  time  to  time,  both  to 
introduce  air  and  to  keep  the  cream  mixed  with  the 
milk.  When  the  proper  acidity  has  been  attained — and 
this  will  vary  with  each  kind  of  cheese,  and  must  be 
determined  by  chemical  means  before  the  cheese- 
making  is  embarked  upon — the  milk  is  then  brought 
to  a  particular  temperature  and  mixed  with  a  certain 
quantity  of  rennet.  Speaking  generally,  the  more  acid 
the  milk,  the  higher  the  temperature  at  which  the 
rennet  is  added  ;  and  the  more  rennet  is  added,  the  more 
quickly  will  the  curd  come  and  the  firmer  consistency 
will  it  attain.  The  necessary  degrees  of  acidity,  the 
most  desirable  temperature,  and  the  proper  strength  of 
the  rennet  have  been  worked  out  for  most  of  the  cheeses 
made  on  a  large  scale.  When  the  curd  has  formed  the 
vat  is  left  to  stand  until  the  curd  begins  to  separate  from 
the  serum  or  whey.  The  curd  consists  of  the  casein,  in 
which  are  entangled  the  globules  of  fat  contained  in  the 
original  milk ;  the  whey  contains  the  albumen  of  the 
original  milk,  all  the  lactose,  and  a  certain  proportion  of 
the  salts,  some  of  the  calcium  phosphate  being  retained 
in  the  casein  in  the  curd.  The  next  stage  in  cheese- 
making  consists  in  getting  up  a  suitable  consistency  in 
the  curd.  In  the  case  of  a  hard  cheese  like  Cheddar, 
the  curd  is  allowed  to  contract  or  shrink  in  the  whey. 
It  is  then  cut  up  to  facilitate  the  removal  of  whey,  and 
the  whole  mass  is  raised  in  temperature  to  increase  still 
further  the  consistency  of  the  curd  and  expel  the  whey. 
Finally  the  curd  is  lifted  out,  allowed  to  drain,  and  put 
through  the  mill  in  order  to  reduce  it  to  small  pieces. 
During  all  these  processes  the  lactic  acid  bacteria  are 
actively  at  work,  and  the  curd  becomes  more  and  more 


286  MILK,  BUTTER,  AND  CHEESE  [chap. 

acid.  In  the  making  of  softer  cheeses  like  Stilton,  the 
curd  is  formed  at  a  lower  temperature  and  is  not  allowed 
to  contract  so  much  in  the  whey,  but  is  removed  and 
placed  in  the  cheese  moulds  when  still  in  a  soft  condition 
and  before  it  has  developed  much  acidity.  In  all  cases, 
however,  the  curd,  when  it  has  reached  its  appropriate 
consistency,  is  packed  into  tin  vessels  lined  with  a  thin 
cloth,  and  perforated  at  the  sides  to  allow  of  the 
expulsion  of  the  whey.  At  this  stage,  too,  a  certain 
amount  of  salt  is  usually  mixed  with  the  curd.  In 
making  Cheddar  cheese,  a  considerable  pressure  is  then 
applied  to  the  curd  in  the  moulds,  and  this  pressure  is 
continued  two  or  three  days  until  the  mass  is  thoroughly 
consolidated.  In  the  softer  cheeses  no  pressure  is 
applied,  and  the  mass  of  curd  has  to  consolidate  by  its 
own  weight.  The  cheese  is  now  made  and  is  a  soft 
mass  possessing  a  slightly  sour  flavour,  the  true  cheese 
flavour  is  only  developed  during  the  ripening.  The 
ripening  processes  which  cheese  undergoes  are  extremely 
various,  thus  giving  rise  to  the  special  and  very  distinct 
flavours  which  the  different  kinds  of  cheese  possess. 
We  may  distinguish  three  distinct  processes  going  on. 
I.  Certain  enzymes  contained  in  the  original  milk 
attack  the  casein  and  gradually  soften  or  even  liquefy 
it,  breaking  it  down  into  a  number  of  simpler  nitrogen 
compounds  of  the  amino-acid  type,  some  of  which  are 
strongly  flavoured. 

2.  Though  the  lactic  acid  bacteria  die  out,  other 
bacteria  develop,  and  some  of  them  form  highly 
characteristic  flavouring  products  out  of  the  casein  or  the 
butter  fat.  In  some  case  special  bacteria  are  introduced 
into  the  curd,  as  in  the  making  of  Gruyere,  Dutch,  and 
Roquefort  cheeses.  Sometimes  the  bacteria  associated 
with  the  special  flavour  of  the  cheese  also  give  rise  to 
gas,  and  blow  round  holes  in  the  cheese,  as  in  Gruyere. 


XIV.]  THE  RIPENING  OF  CHEESE  287 

3.  Certain  moulds,  such  as  Penicillium,  the  blue 
mould  of  cheese,  begin  to  grow  in  the  curd,  and  these 
likewise  split  up  the  casein  and  the  butter  fat,  forming 
from  them  bodies  possessing  strong  flavour  and  smell. 
The  particular  set  of  organisms  which  will  develop  in 
any  make  of  cheese  depends  entirely  upon  the  processes 
of  manufacture,  and  in  many  cases  upon  the  actual 
place  in  which  the  manufacture  is  carried  out.  The 
walls  and  the  vessels  of  dairies  which  have  for  a  long 
time  been  devoted  to  the  manufacture  of  a  particular 
kind  of  cheese  become  impregnated  with  the  organisms 
associated  with  that  cheese  and  communicate  them  to 
the  new  curd.  Furthermore,  the  many  details  in  the 
management  which  experience  alone  has  taught  the 
cheesemaker,  can  be  shown  to  bring  about  the 
encouragement  or  depression  of  particular  groups  of 
organisms  which  affect  the  flavour  of  the  resulting 
cheese.  The  curing  process  is  also  accompanied  by  a 
gradual  shrinking  of  the  cheese  and  an  expulsion  of  the 
whey  which  is  still  contained  in  the  curd.  For  this 
reason  it  is  necessary  to  change  the  cloths  surrounding 
the  cheese  rather  frequently  at  first,  and  to  turn  the 
cheeses  constantly  when  they  have  been  put  into  the 
storage-room.  It  is  also  necessary  to  maintain  the 
storage-room  at  the  constant  temperature  which 
experience  has  shown  to  be  appropriate  to  the  develop- 
ment of  the  special  flavour  of  the  cheese.  As  the 
manufacture  of  cheese  is  thus  dependent  upon  the 
development  of  particular  groups  of  organisms,  it  is  clear 
that  it  may  very  easily  be  turned  in  a  wrong  direction, 
should  any  of  the  encroaching  putrefactive  or  otherwise 
undesirable  organisms,  which  are  abundant  in  dust, 
dirt,  foul  water,  etc.,  establish  themselves  in  the  curd. 
It  is  to  these  intrusive  organisms  that  defects  in  the 
flavour  or  texture  of  cheese  are  usually  due,  and  they 


288  MILK,  BUTTER,  AND  CHEESE  [chap. 

can  only  be  avoided  by  careful  attention  to  cleanliness 
and  to  the  purity  of  the  materials  employed.  The 
wrong  kind  of  organisms  may  have  already  obtained  a 
hold  in  the  milk  itself  at  starting,  generally  through 
want  of  cleanliness  in  the  milking.  Next,  they  may  be 
introduced  by  imperfect  cleaning  of  the  utensils  in  which 
the  milk  stands ;  the  water  used  is  also  often  a  source 
of  danger,  and  has  frequently  been  found  to  be  a  cause 
of  various  troubles  in  the  final  product.  When  natural 
souring  is  trusted  to,  a  not  very  satisfactory  flora  of  the 
various  lactic  acid  making, bacteria  may  become  estab- 
lished in  the  dairy.  For  this  reason  it  is  sometimes 
customary  both  in  cheese-  and  butter-making  on  a  large 
scale  to  begin  by  pasteurising  the  milk  ;  that  is,  by  heating 
it  up  to  a  temperature  of  about  170°  R,  which  kills 
practically  all  the  bacteria  present.  After  cooling,  the 
pasteurised  milk  is  then  mixed  with  a  "starter,"  con- 
sisting either  of  a  pure  culture  of  the  desired  bacteria  or 
of  the  good  but  mixed  culture  contained  in  the  butter- 
milk or  the  sour  whey  derived  from  a  previous  churning, 
or  from  another  dairy  turning  out  products  of  an 
appropriate  flavour.  In  this  way  the  development  of 
acidity  in  the  milk  can  be  controlled,  and  this  is  the 
first  step  towards  obtaining  a  standard  product. 

In  all  the  farmer's  dealingswith  milk  and  its  products  it 
is  important  to  remember  that  the  milk  itself  provides  an 
almost  ideal  feeding-ground  for  bacteria ;  and  as  bacteria 
of  all  kinds  are  particularly  prevalent  in  dust  and  dirt, 
specially  in  the  organic  dirt  that  collects  in  cow  stalls 
and  cattle  sheds,  or  in  a  dairy  which  is  not  kept  most 
scrupulously  clean,  milk  will  deteriorate  more  readily 
than  almost  any  other  natural  product.  The  only  way 
to  overcome  this  difficulty  is  to  exercise  the  most 
scrupulous  cleanliness,  and  to  keep  the  temperature  as 
low  as  possible.     In  the  case  of  milk  and  milk  products, 


XIV.]  CLEANLINESS  289 

dirt  does  mean  disease,  sometimes  even  disease  for 
those  who  consume  the  products ;  more  often,  perhaps, 
the  dirt  gives  rise  to  defects  lowering  the  value  of  those 
products.  To  ward  off  the  growth  of  these  foreign 
bacteria  recourse  should  not  be  had  to  antiseptics 
which  inhibit  the  development  of  bacteria.  The  proper 
remedy  is  cleanliness,  and  cleanliness  alone  is  sufficient 
for  practical  purposes. 


INDEX 


Acid,  sap,  49  ;  vegetable,  75  ;  soils, 
90,  123,  147  ;  manures,  147  ;  soil 
water  in  peaty  soils,   183  ;  fatty, 

171.  173 
Adulteration  of  foods,  180 
Air,  required   in  germination,    li  ; 

carbon  dioxide  in,  22  ;  in  the  soil, 

152 
Albumen,  272,  285 
Albuminoid  ratio,  206,  218 
Albuminoids,  66,  70,  71,  125,  169, 

173,  177 

Albumoses,  173 

Alcohol,  75 

Alfalfa,  138,  141,  168,  199 

Alimentary  canal,  171 

Alkali  land,  139 

Alluvial,  soils,  86,  97,  166 ;  sub- 
soils, 95 

Alumina  in  soils,  94 

Amides,  6,  170,  204 

Amino-acids,  170,173,204,  276,  286 

Ammonia,  absorbed  by  plants,  51  ; 
in  soil,  93  ;  in  nitrification,  I22» 
125  ;  as  a  product  of  putrefaction* 
125  ;  sulphate  of,  250 

Analysis,  of  soil,  mechanical,  88  ;  of 
various  arable  soils,  96 

Antiseptics,  289 

fl-proteins,  6  ;  converted  to  protein, 
66,  70;  proteins  converted  to,  71, 
177 ;    in    animal   food,    175 ;    in 
silage,  177  ;  in  root  crops,  183 
291 


Artificial  manures,  225,  248  ;  nitro- 
genous, 250 ;  phosphatic,  254  ; 
potassic,  257  ;  valuation  of,  267  ; 
composition  of,  268 

Ash,  in  plants,  2  ;  of  crops,  com- 
position of,  55  ;  in  foods,  169 ; 
digestion  of,  175 

Asparagin  absorbed  by  plants,  51 

Aspect  of  land,  117 

Assimilation,  25  ;  potash  required 
for,  261 

Available,  plant  food,  151  ;  energy 
converted  into  mechanical  work, 
198 ;  energy  of  various  foods, 
199 

Azotobacter,  141 

Bacteria,  nitrifying,  122,  145 ; 
putrefactive,  125,  145 ;  ammonia 
producing,  125, 145  ;  denitrifying, 
126,  146 ;  aerobic,  129,  146 ; 
anaerobic,  130,  146  ;  nodule,  132  ; 
nitrogen  fixing,  141  ;  conditions 
necessary  to  growth  of  soil,  145  ; 
in  intestinal  tract,  172,  174  ;  in 
farmyard  manure,  228  ;  in  milk, 
273  ;  cheese,  286 ;  in  the  dairy, 
288 

Barley,  manurial  requirements  of, 
162,  263  ;  as  food,  182 

Barren  soils,  99 

Basalt,  78,  80 

Basic  slag,  256 


292 


INDEX 


Bean,  germination  of,  7  ;  root  of  the, 
42  ;  as  a  recuperative  crop,  138  ; 
soya,  180  ;  as  food,  182 

Biennials,  food  storage,  69 

Bile,  171,  172,  176 

Blood,  the  circulating  medium,  171, 
173,  202 

Bone  meal,  254 

Bones^  254 

Boussingault,  carbon  balance-sheet, 
21  ;  on  leguminous  crops,  135 

Brick  earth,  83 

Bulbous  plants,  59 

Bullocks,  on  maintenance  diet,  195, 
197  ;  on  fattening,  195  ;  minimum 
protein  requirements,  203 

Butter,  281  ;  fat,  271,  276 

Cakes,  oil,  178 

Calf,  composition  of  fat,  207 

Calorie,  186 

Capillarity,  104 

Carbohydrates,  in  plants,  5  ;  in 
foods,  169;  digestion  of,  172; 
proteins  and  fats  in  terms  of,  183  ; 
dependence  on  potash,  261 

Carbon,  in  plants,  2  ;  increased 
weight  of  plants  largely  made 
of,  20;  drawn  from  the  air,  21  ; 
in  humus,  27 

Carbon  dioxide,  evolved  in  germina- 
tion of  seeds,  11  ;  in  air,  22  ;  split 
up  by  plants,  22 ;  excreted  by 
roots,  48,  50  ;  dissolved  in  rain- 
water, 79;  in  soil  gases,  153; 
product  of  combustion  in  animal 
body,  172,  173,  174 

Carbonate  of  lime,  dissolved  by  soil 
water,  80  ;  in  soils,  90  ;  in  sub- 
soil, 95  ;  in  heavy  soils,  beneficial 
effect  of,  98,  167  ;  absence  in  soil 
inhibits  bacterial  action,  144,  147 

Carcass  weight,  208 

Casein,  269,  272,  285 

Castor  cake,  260 


Cellulose,  in  plants,  5  ;  fermentation 
by  soil  bacteria,  129 ;  digestion 
of,  172 

Cereals,  as  foods,  182 

ChaflF,  64 

Chalk,  formations,  97 ;  soils,  90, 
167 

Cheese,  284  ;  ripening  of,  286 

Chewing  of  the  cud,  172 

Chlorine,  in  plants,  50  ;  in  soils,  94 

Chlorophyll,  assimilation,  24 ;  iron 
necessary  to  formation  of,  51 

Churning,  281 

Citric  acid  as  a  soil  solvent,  154 

Clay,  derived  from  felspar  and 
basalt,  80  ;  obtained  in  mechanical 
analysis,  89  ;  properties  of,  91  ; 
soils,  97,  106,  108,  166 

Clover,  on  succeeding  crop,  effect  of 
growth  of,  135  ;  manures  for,  265 

Colostrum,  277 

Colour  of  soils,  116 

Combustion  in  animal  body,  173 

Compensation  for  food  -  stuflfs  con- 
sumed, 243 

Condition  of  the  soil,  147,  239 

Cooking  does  not  increase  digesti- 
bility, 177 

Corm  of  colchicum,  60 

Corn,  place  in  diet,  194,  200 

Cotton -seed  cake,  169,  178,  180, 
183,  192 

Cotyledons,  7 

Cows,  breeds  of,  270,  274 

Cream,  281 

Crocus,  contractile  root  of,  44 ; 
autumn  (colchicum),  60 

Crops,  water  required  by,  39 ;  dry- 
ing action  of,  39 ;  eff'ect  of  catch, 
40  ;  composition  of,  57 

Cultivation,  107 

Curd  of  milk,  272,  284 

Cuticle,  37 

Cuttings,  72 

Cyanamide,  250 


INDEX 


293 


Cytase,  172 

Dandelion,    contracted    root   of, 

44 

Decay  of  organic  matter,  129 

Denitrification,  126 

Desiccation,  66 

Dew,  38 

Diastase,  16,  72  ;  function  in  leaf, 
28  ;  in  saliva,  172 

Diet,  in  digestibility  experiment, 
176  ;  maintenance,  189 

Digestibility,  of  foods,  175  ;  experi- 
ments, 176 

Digestion,  170;  work  expended  on, 

190 

Diminishing  returns,  law  of,  263 
Diphenylamine,  test  for  nitrates,  93 
Disease,  susceptibility  of  plants  to, 

261 
Dormant  plant  food,  151 
Drainage,  I15 
Drift  soils,  82  ;  glacial,  84 
Dry  farming,  113 
Dung,  short  and  long,  226  ;  London, 

236 
Dynamic  energy,  192,  198 

Early  soils,  116 

Earth-nut  cake,  180 

Embryo  in  seed,  7 

Endosperm,  8,  64 

Energy,  stored  by  plants,  33  ;  in 
animal  body,  r72, 173, 185  ;  trans- 
formed to  heat  and  work,  185  ; 
available  for  work,  190,  199 ; 
dynamic,  190,  192  ;  thermal,  188, 
192  ;  total,  187,  192 

Enz)rmes,  16,  72,  76,  171,  203,  272, 
286 

Equivalents,  starch,  184,  215 

Essential  oils,  6 

E^ch'ng,  action  of  roots.      4 

Ether,  in  fat  extraction,  169 

Ethers,  75 


Evaporation,  cooling  effect  of,  109  ; 
loss  of  water  by,  109,  no  ;  checked 
by  hoeing,  in  ;  checked  by 
wind  breaks,  1 14 

F.ECES,  171,  174,  226 

Fallow  on  soil  moisture,  effect  of, 
40,  112 

Farmyard  manure,  158,  224;  com- 
position, 234 ;  use  of,  238,  242  ; 
duration  of,  240  ;  physical  effects 
of,  241  ;  cost  of,  246 

Fats,  in  plants,  5  ;  in  foods,  169  ; 
digestion  of,  171  ;  heat  value  of, 
188  ;  in  terms  of  carbohydrates, 
183  ;  fuel  value  of,  187  ;  in  milk, 
270 

Fattening,  animals,  195,  207  ;  com- 
position of  increase  in,  208 

Fehling's  solution,  15 

Felspar,  80 

Fermentation,  75  ;  aerobic,  128, 129  ; 
anaerobic,  129,  130;  of  farmyard 
manure,  228 

Ferments,  16 

Fern,  growing  in  air-tight  bottle,  31 

Fertilisers,  248  ;  nitrogenous,  250  ; 
phosphatic,  254 ;  potassic,  257  ; 
nitrogenous,  effect  of,  260  ;  phos- 
phatic, effect  of,  261  ;  potassic, 
effect  of,  257  ;  valuation  of,  267  ; 
composition  of,  268 

Fertility,  maintenance  of,  158 

Fibre,  in  plants,  5,  69 ;  in  foods, 
169  ;  digestion  of,  172,  174 

Film,  in  soils,  the  water,  lOl 

Fine  earth,  86 

Finger-and-toe,  147,  256 

Fish  manures,  259 

Fixation  of  nitrogen,  131 

Flocculation,  92,  108,  115 

Food,  used  as  fuel,  172,  174  ;  more 
digestible  than  poor,  rich,  177  ; 
valuation  of,  183  ;  as  a  source  of 
energy,  185  ;  fuel  value  of,  187  ; 


294 


INDEX 


in  relation  to  live  weight  increase, 

210;  rations,  217;  manure  value 

of,  243 
Frost,  distribution  of  plants  by,  47  ; 

disintegration  of  rocks   by,   81  ; 

valleys  specially  liable  to,  117 
Fuel  value  of  food,  187 
Fungi  in  the  soil,  147 

Gases,  soil,  153 

Gastric  juice,  171,  173 

Gelatin  from  bones,  254 

Germination,  conditions  necessary 
to,  9 

Gluten,  6,  65  ;  meal,  181,  183 

Glycerin,  271 

Glycogen,  173 

Grain,  the  filling  of  the,  63 

Granite,  78,  80 

Grass,  composition  of  meadow,  4  ; 
land,  manures  for,  265  ;  character- 
istic, 164,  165,  167 

Gravel,  deposition  of,  82,  83 

Greaves,  259 

Growth,  elements  necessary  to  plant, 
5 1 ;  changes  of  composition  during, 
59  ;  temperature  required  for,  114 

Guanos,  257 

Hay,  growth  and  ripening  of 
meadow,  68  ;  digestibility  of,  177  ; 
composition  of,  182  ;  heat  value 
of,  189  ;  dynamic  value  of,  194, 
199  ;  place  in  diet,  194  ;  values, 
Thaer,  217  ;  manures  for,  265 

Heat  value  of  foods,  188 

Heating  of  soils,  114,  116 

Heavy  soils,  97,  1 18 

Hellriegel,  on  transpiration,  38  ; 
and  Wilfarth  on  nitrogen  fixation, 
132 

Hoeing,  112 

Hops,  74 

Horse,  fed  upon  straw,  191  ;  on 
maintenance  diet,  194, 197  ;  avail- 


able energy  of  various  foods  fed 
to  the,  199  ;  energy  requirements 
of  working,  200 ;  minimum  pro- 
tein requirements,  203 

Humus,  86  ;  soluble  in  alkalis, 
87 ;  in  subsoil,  95 ;  product  of 
cellulose  fermentation,  129,  130; 
value  of,  241 

Hydrochloric  acid,  solvent  action  on 
soils,  94  ;  in  stomach,  175 

Hydrogen,  173,  187 

Ice  as  a  transporting  agency,  84 
Inoculation    of    nodule    organisms, 

132,  139 
Intestines,  171,  176 
Iron,  in  plants,  50  ;  in  rocks,  80  ;  in 

soils,  94 

Kainit,  159,  257 

Kellner,  on  fuel  values,  188,  191  ; 
on  the  maintenance  of  store 
bullocks,  197 ;  energy  require- 
ments of  working  horses,  200 ; 
standard  rations,  223 

Kelp,  257 

Kephir,  273 

Kidneys,  174,  202 

King  on  transpiration,  38 

Koumiss,  273 

Lactation,  period  of,  277 

Lactic  acid,  273,  283,  285 

Lactose,  269,  272 

Lawes  and  Gilbert,  255  ;  ori  trans- 
piration, 38  ;  on  the  composition 
of  farm  animals,  207 

Leaf,  work  of  the,  19  ;  starch  forma- 
tion in  the,  26 ;  removal  of,  34 ; 
stomata  in,  37  ;  the  fall  of 
the,  73 

Lean  meat,  174,  189,  209 

Leguminous  crops,  54,  132,  135  ; 
plants  and  soils,  164,  165,  168 

Light,  assimilation  and,  24,  26 ; 
soils,  98,  116,  118 


INDEX 


295 


Lime,  in  plants,  50,  73  *»  in  ash  of 
crops,  54 ;  in  soil?,  94 ;  nitrate 
of,  250,  253  ;  in  basic  slag,  250 

Limestones,  97 

Linseed  cake,  176,  178,  181 

Lipase,  16,  171 

Litter,  228 

Liver,  173 

Loam,  107,  165 

London  dung,  236 

Lucerne,  138,  141,  168,  199 

Lymphatic  system,  171 

Magnesia,  in  plants,  50 ;  in  soils, 

94 
Maintenance  diet,  189,  195 
Maize,  gluten  meal,  182  ;  as  food, 

182;     d)mamic    value    of,    1 99; 

manures  for,  264 
Malt,  16 
Mangold,     yield     proportional    to 

combined  nitrogen  supplied,  53  ; 

food  storage,  69  ;  composition  of, 

69,  182  ;  manures  for,  264 
Manure,  farmyard,   224 ;   artificial, 

325,  248  ;  required  by  crops,  263  ; 

value  of  foods,  243 
Manuring,  theories  of,  161 
Marl,  98,  167 

Marsh-gas,  127,  173,  187,  229 
Meals,  178 
Meat  guanos,  259 
Metabolism,  of  proteins,  66,  70,  71, 

173,  202  ;  of  carbohydrates,  172  ; 

of  fats,  171  ;  of  starch,  14,26,  172 
Methane,  127,  173,  187  ;   in  farm- 
yard manure,  229 
Mica,  80 
Micropyle,  9 

Migration  of  plant  food,  60,  63 
Milk,  fat  in,  171,  173  ;  composition 

of,   269,    273 ;  souring  of,   273 ; 

morning      and     evening,     278 ; 

analysis,  280 
Moisture  in  foods,  169 


Moulds  in  cheese,  287 
Mulch,  III  ;  soil,  113 
Mummy  wheat,  13 
Miintz  on  digestion,  191 
Muriate  of  potash,  257 
Mustard  seed  in  rape  cake,  178 

Nitrate,  of  soda,  250,  252  ;  of 
lime,  250,  253 

Nitrates,  in  water  cultures,  51  ;  in 
soil,  93, 121  ;  lost  in  denitrification, 
126 

Nitre,  121 

Nitrification,  12 1 

Nitrites  formed  in  process  of  nitri- 
fication, 122 

Nitrogen,  in  plants,  3  ;  required  in 
combined  state  by  plants,  53  ; 
accumulation  in  black  virgin  soils, 
54  ;  in  plant  food,  66,  70 ;  in 
humus,  87 ;  in  soils,  95,  149 ; 
cycle  illustrated,  125  ;  lost  by 
denitrification,  127 ;  fixation  by 
bacteria,  131  ;  losses  and  gains 
during  a  four-course  rotation, 
159;  excreted  by  kidneys,  174; 
losses  of,  in  manure-making,  230  ; 
in  fertilisers,  250 

Nitrolim,  250 

Nodules,  on  roots  of  leguminous 
plants,  132  ;  organisms,  racial 
adaptation  of,  1 39 

Oats,  as  food,  182,  199;  manures 

required  by,  264 
Offal,  weight  of,  208 
Oils,  in  plants,  5  ;  cakes,  169,  178  j 

digestion  of,  171,  174 
Organic  matter,  in  soils,  86,  116  ; 

promotes  denitrification  in  soils, 

126,  146 
Oxen,  composition  of,  207 
Oxygen  evolved  by  plants,  23 

Palm-nut  cake,  180 


296 


INDEX 


Pancreatic  juice,  171,  172,  173 

Papain,  16 

Pastures,  fattening,    195  ;    manures 

for,  265 
Peas  as  food,  182 
Peat,  130  ;  moss  litter,  228  ;  moss 

manure,  236;   soils,   86,  90,  95, 

123, 153,  Its 

Pectins,  71,  75,  182 

Pepsin,  16,  173 

Peptone,  124,  173 

Percolation,  105 

Pericarp,  64 

Peruvian  guanos,  257 

Phosphate  of  lime,  a5  a  food  con- 
stituent, 175 

Phosphoric  acid,  necessary  to  plant 
growth,  50 ;  in  soils,  94,  149 ; 
removed  in  four-course  rotation, 
159;  deficient  in  clays,  167;  in 
fertilisers,  254 

Physical  effects  of  farmyard  manure, 
241 

Pigs,  composition  of,  207 

Plant,  food,  migration  and  storage 
of,  60,  63  ;  food,  dormant  and 
available  in  soil,  151  ;  excretions, 

157 
Ploughing,   autumn,    107 ;    spring, 

108  ;  work  done  by  horses  in,  200 
Pore  space,  104 
Potash,  in  plants,  50,  53  ;  in  soils, 

94,  149 ;  removed  in  four-course 

rotation,  158  ;  abundant  in  clays, 

167  ;  manures,  257 
Potato,    72;    districts,   early,    118; 

composition  of,  182  ;  manures  for, 

257 

Prairie  soils,  144 

Preservatives  in  dung-heaps,  233 

Protease,  16 

Protein,  in  plants,  6  ;  and  a-proteins, 
66,  70,  71,  177  ;  breakdown  and 
reconstruction,  125  ;  in  foods, 
169  ;  digestion  of,  173  ;  in  silage, 


177  ;  heat  value  of,  188,  201  ;  in 
terms  of  carbohydrates,  183  ;  fuel 
value  of,  187  ;  requirements,  174, 
201 

Protozoa,  147 

Puddled  clay,  91 

Putrefaction,  124 

Quality  in  produce,  77 
Quartz,  80 

Rape,  cake,  178  ;  dust,  259 

Rations,  construction  of  food,  217  ; 
for  milch  cows,  281 

Rennet,  272,  285 

Respiration,  in  plants,  29,  60,  63  ; 
of  roots,  47  ;  during  storage  of 
root  crops,  71  ;  chamber,  211 

Ripening,  of  grain,  66  ;  of  fruits, 
57 

Rock,  78  ;  primitive,  80  ;  volcanic, 
80  ;  phosphate,  255 

Rolling,  no 

Root,  22  ;  hairs,  43  ;  adventitious, 
44 ;  source  of  water  supply  to 
plants,  46  ;  effect  of  frost  on,  47  ; 
absorbs  soluble  substances,  48 ; 
etching  action  of,  49  ;  part  played 
in  weathering,  81  ;  nodules,  132 

Root  crops  during  storage,  changes 
in,  71  ;  digestibility  of,  178  ;  com- 
position of,  182 

Rotations,  value  of,  1 56 

Ruminants,  172,  174,  177 

Sainfoin,  138,  168 

Saliva,  172 

Salt  as  a  food  constituent,  175 

Sand,  deposition  of,  82,  83  ;  in 
mechanical  analysis,  88 ;  pro- 
perties of,  91  ;  in  foods,  170 

Sandy  soils,  98,  107,  163 

Sap,  50  ;  acidity,  49 

Sea,  temperature  in  proximity  to, 
118 


INDEX 


297 


Seed,  6  ;  "  hard,"  9 ;  vitality  of, 
13  ;  depth  of  sowing,  13  ;  test- 
ing, 17 

Seed-bed,  18,  108,  113,  241 

Separation  of  milk,  281 

Sheep,  on  maintenance  diet,  195, 
197 ;  on  fattening  diet,  195  ; 
minimum  protein  requirements, 
203  ;  composition  of,  207 

Shoddy,  260 

Silage,  digestibility  of,  177 

Silica,  in  plants,  50,  54,  73  ;  in  soil, 

94 

Silt,  deposition  of,  83 

Slag,  basic,  256 

Soda,  in  plants,  50 ;  in  soils,  94  ; 
nitrate  of,  250,  252 

Soil,  origin  of,  78  ;  sedentary,  79  ; 
of  transport,  82  ;  motion  of  the, 
84  ;  chemical  constituents  of,  93  ; 
relation  to  subsoil,  95 ;  water, 
movements  of,  100 ;  colour  of, 
116;  inoculation,  139;  chemical 
composition  of  the,    149 ;    gases, 

153 

Soot,  116,  250,  253 

Souring  of  milk,  273 

Soya  bean  cake,  180,  181 

Standard  rations,  223 

Starch,  in  plants,  5,  14,  26  ;  iodine 
test,  14;  digestion  of,  172;  heat 
value  of,  188  ;  equivalent,  184, 
215  ;  use  of,  219 

Starters,  milk,  288 

Stassfurt  potash  deposit,  257 

Steppe  soils,  144 

Stomach,  171 

Stomata,  37 

Stones,  growing  on  arable  soils,  84  ; 
in  soil  samples,  86  ;  check  evap- 
oration, 115 

Storage,  of  plant  food,  60  ;  of  fats  in 
animal  body,  173  ;  of  protein  in 
animal  body,  174 

Store  condition,  194,  197 


Straw,  digestibility  of,  177  ;  com- 
position of,  65,  182  ;  food  value, 
191,  199  ;  as  litter,  228 

Strippings,  milk,  278 

Subsoil,  79,  95  ;  packing,  113 

Sugars,  in  plants,  5  ;  test  for,  15  ; 
in  mangolds  and  beet,  69 ;  in 
turnips,  71  ;  action  of  yeast  on, 
76  ;  digestion  of,  171,  174 

Sulphate  of  ammonia,  250  ;  potash, 
257 

Sulphur,  in  plants,  50  ;  in  soil,  94 

Sun,  source  of  heat  to  soils,  114 

Superphosphate,  158,  255 

Surface  tension,  100 ;  of  soil  par- 
ticles, 105 

Swede  turnip,  food  storage,  69 ; 
composition  of,  71,  182  ;  manurial 
requirements  of,  163,  264 

Symbiosis,  133 

Tankage,  259 

Tannins,  75 

Temperature,  soils,  114;  dung-heap, 
230;  body,  196 

Test,  for  starch,  14  ;  for  sugar,  15  ; 
seed,  17 ;  for  carbon  dioxide, 
24 

Texture    of   soil,    108,    115,    241 
252 

Thaer,  217 

Thermal  energy,  193 

Thomas's  phosphate  powder,  256 

Tillering,  44 

Tilth,  108,  157,  252 

Transpiration,  36 ;  devices  to  re- 
duce, 41 

Transport,  soils  of,  84 

Trees,  characteristic,  165,  167,  168 

Trypsin,  173 

Turnips,  food  storage,  69  ;  composi- 
tion of,  182 

Urea,  174, 187,  202,  229 
Urine,  174,  203,  225,  229 


298 


INDEX 


Valuation,  of  feeding  stuffs,  183, 

213;  of  fertilisers,  267 
Vetches,  138 
Virgin  soils,  54,  144. 

Water,  in  plants,  2  ;  required  by 
crops,  39,  107  ;  cultures,  45  ;  as 
a  weathering  agency,  79  ;  as 
a  transporting  agency,  82 ;  in 
animal  body,  171,  172,  174 

Waxes,  5 

Weathering  of  soils,  79 

Weeds,  34  ;  seeds  in  oilcakes,  181 

Wheat,  root,  43  ;  development  of, 
61  ;  effect  of  wet  autumn  on,  62  ; 
"strong,"  65,  156;  in  wet  and 
dry  seasons,  67  ;  manurial  require- 
ments of,  162,  263  ;  as  food,  182 


Whey,  273,  285 

Wilting,  46 

Wind,  as  a  transporting  agency,  82  ; 
breaks,  1 14 

Wollny,  38 

Wood  ashes,  257 

Wool,  203 

Work,  done  in  animal  body,  172, 
I73>  189;  energy  converted  to, 
186  ;  done  in  walking  and  trot- 
ting, 198 

Worms,  action  of,  85 

Yeast,  75 

Zein,  204 

Zuntz,  191,  197,  198 


OLIVER   AND   BOYD,    EDINBURGH. 


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